This study will determine the efficacy and safety of citalopram compared to placebo in the treatment of children with autism.

For children with autism spectrum disorders (ASD, also known as Pervasive Developmental Disorders - PDDs), repetitive behaviors are common and frequently interfere with functioning in the home as well as in social and educational settings. These behaviors may involve repetitive movements, rigid routines, repetitive play, and even repetitive speech. These behaviors may be associated with high levels of anxiety, severe tantrums. Self-injury can occur when these behaviors and routines are interrupted.

Participants will be randomly assigned to receive citalopram or placebo (administered as liquid), and carefully followed every two weeks. At the end of 12 weeks, children who have responded to treatment will be given the opportunity to continue in the study, with monthly visits, for an additional 24 weeks. Children who received placebo and did not respond to treatment at 12 weeks will be given the opportunity to receive a carefully monitored 12 week course of citalopram.

Inclusion Criteria:

• Able to walk
• Diagnosis of Autistic Disorder, Asperger's Disorder, or PDD-NOS
• Have a score greater than or equal to (>) 8 on the sum of items 1A, 2, 3 and 5 of the Compulsions Subscale of the Revised CYBOCS.
• Have a rating of at least moderate behavioral disturbance based on the modified Clinical Global Impression-Severity of Illness score (CGI-S) at the time of screening (See description below).
• Be free of psychotropic medication for at least one month for fluoxetine, two weeks for other SSRIs and neuroleptics, and for 5 days for stimulants prior to baseline ratings.
• Be judged reliable for medication compliance and agree to keep appointments for study contacts and tests as outlined in the protocol (both subject and guardian(s)).

Exclusion Criteria:

• Medical contraindications to therapy with SSRIs
• Prior exposure to citalopram (or escitalopram) of sufficient dose or duration to determine response status
• History of treatment failure to a clinically adequate trial of two select SSRIs
• Diagnosis of Rett's Disorder or Childhood Disintegrative Disorder
• Uncontrolled epilepsy, with a seizure within past 6 months
• Child weighs less than (<) 15 kg at screening contact.
• Pregnancy
• Presence of chronic medical conditions that might interfere with study participation or where study participation would be contraindicated
• Clinically significant abnormal baseline laboratory testing
• History of bipolar disorder or manic episode induced by antidepressant exposure
• Documented need for ongoing psychotropic medications besides study medication (with the exception of stable dose (at least 3 month) anti-convulsants for seizures).
• Concomitant medication that would interfere with participation in the study.
• Recent (< 2 months) initiation of behavior therapy (such as parent training, applied behavior analysis or behavior modification) in a clinic, with a private practitioner or in a school program. Participants who are in an established behavior therapy program (defined as greater than (>) 2 months for clinic or private practitioner or greater than (>) 1 month for school program) can be included in the study. Families will be asked not to initiate any new behavior therapy during the study.