To the knowledge of the investigators there are no studies documenting the pharmacokinetics and pharmacogenetics of risperidone in children with PDD. Currently the PPRU network is conducting a PK study whose aim is to establish a new enantio-selective micro-assay methodology and to generate preliminary population PK data of risperidone and its metabolites in PDD. This study focuses on pharmacogenetic evaluation of PDD patients having little or no effect, those that are unusually sensitive and those experiencing drug toxicity/adverse events at standard risperidone dosages.