 Pervasive developmental disorders PDD are characterized by severe impairments in social interaction and communication in addition to restricted patterns of interests and activities. Research suggests that a dysregulation of the dopamine and serotonin systems contributes to these interfering behaviors in individuals with PDD. After benefits of typical neuroleptics were reported in subjects with PDD, research shifted to the atypical antipsychotic which have been shown to be better tolerated and effective in this population. However, the atypical antipsychotics have also been associated with adverse effects. Thus there remains a need for a novel pharmacotherapy that would be safe and effective for children and adolescents with PDD. The primary objectives of this study is to determine whether aripiprazole is effective and well tolerated for irritability in children and adolescents with PDD NOS during an 8-week acute phase and whether the effectiveness and tolerability of aripiprazole is maintained during a 16-week continuation phase. 