Objective: Children with Pervasive Developmental Disorders (PDDs) have social interaction deficits, delayed communication, and repetitive behaviors as well as impairments in adaptive functioning. Many children actually show a decline in adaptive skills compared with age mates over time. Method: This 24-week, three-site, controlled clinical trial randomized 124 children (4 through 13 years of age) with PDDs and serious behavioral problems to medication alone (MED; n = 49; risperidone 0.5 to 3.5 mg/day; if ineffective, switch to aripiprazole was permitted) or a combination of medication plus parent training (PT) (COMB; n = 75). Parents of children in COMB received an average of 11.4 PT sessions. Standard scores and Age-Equivalent scores on Vineland Adaptive Behavior Scales were the outcome measures of primary interest. Results: Seventeen subjects did not have a post-randomization Vineland assessment. Thus, we used a mixed model with outcome conditioned on the baseline Vineland scores. Both groups showed improvement over the 24-week trial on all Vineland domains. Compared with MED, Vineland Socialization and Adaptive Composite Standard scores showed greater improvement in the COMB group (p = 0.01 and 0.05, and effect sizes = 0.35 and 0.22, respectively). On Age Equivalent scores, Socialization and Communication domains showed greater improvement in COMB versus MED (p = 0.03 and 0.05, and effect sizes = 0.33 and 0.14, respectively). Using logistic regression, children in the COMB group were twice as likely to make at least 6 months' gain (equal to the passage of time) in the Vineland Communication Age Equivalent score compared with MED (p = 0.02). After controlling for IQ, this difference was no longer significant. Conclusion: Reduction of serious maladaptive behavior promotes improvement in adaptive behavior. Medication plus PT shows modest additional benefit over medication alone. Clinical trial registration information--RUPP PI PDD: Drug and Behavioral Therapy for Children With Pervasive Developmental Disorders; http://www.clinicaltrials.gov; NCT00080145. (Contains 4 tables.)