This study compares divalproex sodium and placebo in the treatment of autistic disorder. Twenty six child or adolescent outpatients, with age ranges from 5-17, will be randomized into a 12-week double-blind, placebo-controlled parallel treatment study. During the 12 weeks, patients will be monitored by the treating psychiatrist and assessed by an independent evaluator (IE). The IE will perform study assessments while remaining blind to medication regimens (including possible tapering) as well as any side effects. Study assessments will be administered at designated time points