• Mean Clinical Global Impressions Improvement Scale (CGI-I) Score [ Time Frame: Week 8 ] [ Designated as safety issue: No ]
• Number of Participants With Response at Week 8 [ Time Frame: Week 8 ] [ Designated as safety issue: No ]
• Mean Change (Week 8 - Baseline) in the Children's Yale-Brown Obsessive Compulsive Scale (CY-BOCS; Compulsion Scale Only) [ Time Frame: Week 8 ] [ Designated as safety issue: No ]
• Mean Change (Week 8 - Baseline) in the Other ABC Subscale Scores [ Time Frame: Week 8 ] [ Designated as safety issue: No ]
• Mean Change (Week 8 - Baseline) in CGI-Severity (CGI-S) [ Time Frame: Week 8 ] [ Designated as safety issue: No ]
• Summary of Safety [ Time Frame: continuous throughout the study ] [ Designated as safety issue: Yes ]
• Change From Baseline in Body Weight [ Time Frame: Week 8 ] [ Designated as safety issue: Yes ]

This study will compare the effectiveness (how well the drug works) of aripiprazole, flexibly dosed with a placebo, in reducing serious behavioral problems in children and adolescents with a diagnosis of autistic disorder (AD).

• Drug: Aripiprazole
• Drug: Placebo

Inclusion Criteria:

• Meets current Diagnostic and Statistical Manual for Mental Disorders-Fourth Edition (DSM-IV) diagnostic criteria for AD and demonstrates serious behavioral problems. Diagnosis confirmed by Autism Diagnostic Interview- Revised (ADI-R)
• CGI score > = 4 AND and Aberrant Behavior Checklist (ABC) Irritability/Agitation subscale score > = 18 at screening and baseline (randomization)
• Mental age of at least 18 months
• Male or female 6 to 17 years of age, inclusive, at the time of randomization

Exclusion Criteria:

• Patients considered treatment resistant to neuroleptic medication based on lack of therapeutic response to 2 different neuroleptics after treatment of at least 3 weeks each.
• Patients previously treated and not responding to aripiprazole treatment
• The patient is currently diagnosed with another disorder on the autism spectrum, including PDD-NOS, Asperger's Disorder, Rett's Disorder, Fragile-X Syndrome or Childhood Disintegrative Disorder
• Current diagnosis of bipolar disorder, psychosis, or schizophrenia, or major depression
• A seizure in the past year
• History of severe head trauma or stroke
• Patients undergoing non-pharmacologic therapies (e.g., psychotherapy, behavioral modification) must have started at least 2 months prior to the initial screening visit and must remain in a consistent treatment program for the duration of the study.