Objective: To evaluate the short-term efficacy and safety of aripiprazole in the treatment of irritability in children and adolescents with autistic disorder. Method: Two hundred eighteen children and adolescents (aged 6-17 years) with a diagnosis of autistic disorder, and with behaviors such as tantrums, aggression, self-injurious behavior, or a combination of these symptoms, were randomized 1:1:1:1 to aripiprazole (5, 10, or 15 mg/day) or placebo in this 8-week double-blind, randomized, placebo-controlled, parallel-group study. Efficacy was evaluated using the caregiver-rated Aberrant Behavior Checklist Irritability subscale (primary efficacy measure) and the clinician-rated Clinical Global Impressions-Improvement score. Safety and tolerability were also assessed. Results: At week 8, all aripiprazole doses produced significantly greater improvement than placebo in mean Aberrant Behavior Checklist Irritability subscale scores (5 mg/day, -12.4; 10 mg/day, -13.2; 15 mg/day, -14.4; versus placebo, -8.4; all p less than 0.05). All aripiprazole doses demonstrated significantly greater improvements in mean Clinical Global Impressions-Improvement score than placebo at week 8. Discontinuation rates due to adverse events were as follows: placebo 7.7%, aripiprazole 5 mg/day 9.4%, 10 mg/day 13.6%, and 15 mg/day 7.4%. The most common adverse event leading to discontinuation was sedation. There were two serious adverse events: presyncope (5 mg/day) and aggression (10 mg/day). At week 8, mean weight change (last observation carried forward) was as follows: placebo +0.3 kg, aripiprazole 5 mg/day +1.3 kg, 10 mg/day +1.3 kg, and 15 mg/day +1.5 kg; all p less than 0.05 versus placebo. Conclusions: Aripiprazole was efficacious and generally safe and well tolerated in the treatment of children and adolescents with irritability associated with autistic disorder. (Contains 4 tables and 3 figures.)