Many autistic children have associated problems of inattention, impulsivity, and hyperactivity that limit the effectiveness of educational and behavioral interventions. Few controlled psychopharmacologic trials have been conducted in autistic children to determine which agents may be effective for these associated features. Eight male children (8.1 +/- 2.8 years) with autistic disorder, diagnosed by DSM-III-R criteria, completed a placebo-controlled, double-blind crossover trial of clonidine. Subjects were included in the study if they had inattention, impulsivity, and hyperactivity that was excessive for their developmental level. Subjects had not tolerated or responded to other psychopharmacologic treatments (neuroleptics, methylphenidate, or desipramine). Teacher ratings on the Aberrant Behavior Checklist irritability, stereotypy, hyperactivity, and inappropriate speech factors were lower during treatment with clonidine than during treatment with placebo. Attention deficit disorder with hyperactivity: Comprehensive Teacher's Rating Scale ratings were not significantly improved during the study, except for oppositional behavior. Parent Conners Abbreviated Parent-Teacher Questionnaire ratings significantly improved during clonidine treatment. Clonidine led to increased ratings of the side effects of drowsiness and decreased activity. Clinician ratings (Children's Psychiatric Rating Scale Autism, Hyperactivity, Anger and Speech Deviance factors; Children's Global Assessment Scale; Clinical Global Impressions efficacy) of videotaped sessions were not significantly different between clonidine and placebo. Clonidine was modestly effective in the short-term treatment of irritability and hyperactivity in some children with autistic disorder.