Risperidone is an important medication used to treat children with psychiatric illnesses or neurodevelopmental disorders, such as autism. Despite excellent symptom control, the potential for side effects is worrisome. Treating these disorders is difficult because not everyone responds the same way to the same risperidone dose. One reason for this is genetic differences in how people break down the drug. Understanding these differences will help clinicians choose a dose and better predict the response so patients will be treated successfully with a lower risk for side effects. This study will research these genetic differences in children with psychiatric or neurodevelopmental disorders. Hypothesis: The inter-patient variability in risperidone pharmacokinetics and exposure, adverse events, and clinical response in patients with psychiatric or neurodevelopmental disorders is associated with identifiable pharmacogenetic factors, such as CYP2D6 single nucleotide polymorphisms (SNPs).

Inclusion Criteria:

• Previous risperidone PK study participation (CCHMC, Rainbow Babies and Children's Hospital or OSU)
• CYP2D6 PM predicted phenotype
• Actively taking risperidone
• Under 18 years of age at time of enrollment
• Signed, dated informed consent forms

Exclusion Criteria:

• Investigators are unable to contact the subject/legal guardian(s)
• Subject is no longer taking risperidone
• CYP2D6 predicted phenotype other than PM
• Subject is non-White, with respect to race, for PK study participation
• Subject is 18 years of age or older
• Subject is less than 5 years of age
• Subject is pregnant at the time of the full PK study
• Subject/legal guardian unwilling or unable to participate in the study

Subjects will include up to 41 risperidone-treated children and adolescents with psychiatric or neurodevelopmental (ND) disorders who participated in one of two previous risperidone pharmacokinetics investigations.

To have a larger sample of poor metabolizer subjects, we plan to enroll at least 8 additional subjects who are on stable treatment with risperidone, and perform risperidone pharmacokinetics analyses. Prospectively enrolled subjects (n = 8) will be CYP2D6 poor metabolizers and will include White / Caucasian subjects, of any socioeconomic status, and will be recruited from inpatients or outpatients at Cincinnati Children's Hospital