 - Participants: must meet the study definition for diagnosis of autistic disorder as determined by clinical diagnosis based upon DSM-IV criteria, the Autism Diagnostic Interview-Revised ADI-R and the Autism Diagnostic Observation Schedule ADOS performed at baseline 1. ADI-R will be conducted by trained study staff at Baseline 1 Visit. If the participant has had an ADI-R in the past 12 months, this will be accepted provided the person administering and scoring the test is site personnel validated for the study. 