Participants assigned to placebo will take a placebo pill for the initial 8 weeks of treatment.

D-cycloserine will be dosed in the range of 25 to 200 mg daily for the final 8 weeks of treatment.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicate greater severity. Four item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total inappropriate speech scale score ranging from 0 to 12. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicates greater severity. Sixteen item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total social withdrawal scale score ranging from 0 to 48. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicates greater severity. Seven item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total stereotypy scale score ranging from 0 to 21. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

The CYBOCS- PDD is a semi-structured clinician-rated scale designed to rate the current severity of repetitive behavior in children with idiopathic autism spectrum disorders. Once the current repetitive behaviors are identified, they are rated on: Time Spent, Interference, Distress, Resistance, and Control. Each of these items is scored from 0 (least symptomatic) to 4 (most symptomatic), yielding a Total score from 0 to 20. Higher scores indicate higher severity. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

The Vineland Adaptive Behavior Scales, (Survey Interview Form) is a measure of adaptive behavior in children, adolescents and adults. It yields an overall standard score (Adaptive Behavior Composite, ABC) and age standard scores in four domains. ABC scores have a mean of 100 and a standard deviation of 15 (range = 20 to 160). Higher scores suggest a higher level of adaptive functioning. An increase in standard scores from Baseline to the Final Visit indicates improvement. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates.

Autism Diagnostic Observation Scale (ADOS) (Lord), Modules 1-3 yields scores in 2 scales: Social Affect and Restricted and Repetitive Behavior. The ADOS has been repeatedly evaluated as a diagnostic measure, it has also been used as an outcome measure of autism severity (Aldred et al., 2004; Gutstein, 2007; Owly et al, 2001, Green et al, 2010). For modules 1-3, scores ranging from 0-2 on 14 items are summed to arrive at the Social Affect and Restricted and Repetitive Behavior Total Score, which ranges from 0 to 28. Higher scores indicate greater autism severity. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates

The 65-item SRS is a standardized measure of the core symptoms of autism. Each item is scored on a 4-point Likert scale. The score of each individual item is summed to create a total raw score. A total score results are as follows: 0-62: Within normal limits 63-79: Mild range of impairment 80-108: Moderate range of impairment 109-149: Severe range of impairment. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates.

Total of all reporting groups

Clinical Global Impressions (Guy, 1976) global improvement (CGI-I) is designed to take into account all factors to arrive at an assessment of response to treatment. The CGI-I scale ranges from 1 to 7, with lower scores indicating greater improvement (1=very much improved and 2=much improved). Participants with a CGI-I score of 1 or 2 were classified as improved. Odds were estimated using a repeated measures logistic regression model with treatment group, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average log odds across study timepoints. Confidence intervals reflect a Bonferroni multiple testing correction accounting for the selection of two primary outcomes.

All randomized study participants with at least one post-baseline CGI-I rating. One participant assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline ratings.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. The 16-item Hyperactivity subscale covers overactivity, impulsiveness, inattention and noncompliance. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscale is that higher scores, indicate greater severity. Sixteen item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total score ranging from 0 to 48. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

All randomized study participants with at least one post-baseline ABC score. One participant assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline assessments.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicate greater severity. Four item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total inappropriate speech scale score ranging from 0 to 12. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

All randomized study participants with at least one post-baseline ABC score. One participant assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline assessments.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicates greater severity. Sixteen item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total social withdrawal scale score ranging from 0 to 48. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

All randomized study participants with at least one post-baseline ABC score. One participant assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline assessments.

The Aberrant Behavior Checklist (ABC) is a symptom checklist for assessing problem behaviors in individuals ages 6 to 54 with mental retardation. The full ABC is a 58-item Parent rating with five factors: Irritability, Social Withdrawal, Stereotypy, Hyperactivity and Inappropriate Speech. It has been used as a primary outcome measure in several trials of children with developmental disabilities. The interpretation of the tool and its subscales is that higher scores, indicates greater severity. Seven item scores with values ranging from 0 (not a problem) to 3 (problem is severe) are summed to arrive at the total stereotypy scale score ranging from 0 to 21. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

All randomized study participants with at least one post-baseline ABC score. One participant assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline assessments.

The CYBOCS- PDD is a semi-structured clinician-rated scale designed to rate the current severity of repetitive behavior in children with idiopathic autism spectrum disorders. Once the current repetitive behaviors are identified, they are rated on: Time Spent, Interference, Distress, Resistance, and Control. Each of these items is scored from 0 (least symptomatic) to 4 (most symptomatic), yielding a Total score from 0 to 20. Higher scores indicate higher severity. Means were estimated using a repeated measures linear regression model with treatment group, baseline score, study week (in categories), and sex x Tanner stage stratum as covariates. A linear contrast estimated the average across study timepoints.

All randomized study participants with at least one post-baseline CY-BOCS score. Two participants assigned to placebo and 2 participants assigned to aripiprazole had no post-baseline assessments.

The Vineland Adaptive Behavior Scales, (Survey Interview Form) is a measure of adaptive behavior in children, adolescents and adults. It yields an overall standard score (Adaptive Behavior Composite, ABC) and age standard scores in four domains. ABC scores have a mean of 100 and a standard deviation of 15 (range = 20 to 160). Higher scores suggest a higher level of adaptive functioning. An increase in standard scores from Baseline to the Final Visit indicates improvement. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates.

All randomized study participants. For 7 participants who exited the double-blind phase prior to week 8 but completed an exit interview (all assigned to placebo), week 8 scores were imputed using a last observation carried forward approach.

Autism Diagnostic Observation Scale (ADOS) (Lord), Modules 1-3 yields scores in 2 scales: Social Affect and Restricted and Repetitive Behavior. The ADOS has been repeatedly evaluated as a diagnostic measure, it has also been used as an outcome measure of autism severity (Aldred et al., 2004; Gutstein, 2007; Owly et al, 2001, Green et al, 2010). For modules 1-3, scores ranging from 0-2 on 14 items are summed to arrive at the Social Affect and Restricted and Repetitive Behavior Total Score, which ranges from 0 to 28. Higher scores indicate greater autism severity. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates

All randomized study participants. For 9 participants who exited the double-blind phase prior to week 8 but completed an exit interview (8 assigned to placebo and 1 to aripiprazole), week 8 scores were imputed using a last observation carried forward approach.

The 65-item SRS is a standardized measure of the core symptoms of autism. Each item is scored on a 4-point Likert scale. The score of each individual item is summed to create a total raw score. A total score results are as follows: 0-62: Within normal limits 63-79: Mild range of impairment 80-108: Moderate range of impairment 109-149: Severe range of impairment. Means were estimated using a repeated measures linear regression model with treatment group, study week (in categories, baseline or 8 weeks), and sex x Tanner stage stratum as covariates.

All randomized study participants. For 9 participants who exited the double-blind phase prior to week 8 but completed an exit interview (8 assigned to placebo and 1 to aripiprazole), week 8 scores were imputed using a last observation carried forward approach.

Participants will take aripiprazole

Aripiprazole: Participants will receive 8 weeks of initial treatment with aripiprazole. If responsive to treatment, participants may be assigned to an additional 16 weeks of aripiprazole and then an additional 8 more weeks of aripiprazole with D-cycloserine. Dosing schedule for participants less than 50 mg maximum dose will be 10 mg per day. Dosing schedule for participants greater than 50 kg maximum dose will be 15 mg.

Participants first will take aripiprazole then will also take D-cycloserine

Aripiprazole: Participants will receive 8 weeks of initial treatment with aripiprazole. If responsive to treatment, participants may be assigned to an additional 16 weeks of aripiprazole and then an additional 8 more weeks of aripiprazole with D-cycloserine. Dosing schedule for participants less than 50 mg maximum dose will be 10 mg per day. Dosing schedule for participants greater than 50 kg maximum dose will be 15 mg.

D-cycloserine: D-cycloserine will be dosed in the range of 25 to 200 mg daily for the final 8 weeks of treatment.