 This is a multi-center, randomized, placebo-controlled, double-masked study of 166 evaluable participants taking buspirone twice daily for 6 months. Children aged 2-6 years with autism will be randomized to receive one of three treatments: 2.5mg, 5.0mg, or matched placebo. The placebo controlled trial will be followed by an optional follow-up trial to assess the long term safety of buspirone. In addition, a PET scan of serotonin synthesis and plasma serotonin will be measured at baseline to determine whether these measures are predictors of drug response. This trial is aimed at the core features of autism. The outcome measures for efficacy will be examiner and parent ratings on psychological tests and questionnaires. The outcome measure for the primary objective will be the Autism Diagnostic Observation Scale ADOS Composite Total score. The behavioral outcomes for the secondary aims are delineated in the study design. 