Study to Evaluate the Efficacy and Safety of Aripiprazole

Inclusion Criteria: * 6 to 17 years of age. * Meeting the diagnostic criteria for autistic disorder specified by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR), with a diagnosis corroborated by the Autism Diagnostic Interview-Revised (ADI-R) diagnostic instrument and demonstrated behaviors such as tantrums, aggression, self-injurious behavior, or a combination of these. * Clinical Global Impressions (CGI) Severity subscale score ≥ 4 at screening and baseline. * Aberrant Behavior Checklist (ABC) Irritability subscale score ≥18 at screening and baseline. * Mental age ≥ 18 months. * Women of childbearing potential (WOCBP) have to use an adequate method of contraception * WOCBP must have had a negative serum or urine pregnancy test. * The patient and/or the designated guardian(s) or caregiver(s) who are able to comprehend and comply with the protocol requirements, in the opinion of the investigator and have consented to participate by signing an informed consent form. Exclusion Criteria: * Current diagnosis of bipolar disorder, schizophrenia, major depressive disorder, Rett's disorder, or Fragile-X syndrome. * History of neuroleptic malignant syndrome. * Significant risk of committing suicide based on history or routine psychiatric status examination. * History of seizure in the past 1 year. * History of severe head trauma or stroke * History or current evidence of any unstable medical conditions * Patient considered treatment resistant to neuroleptic medication * Patient considered treatment resistant to aripiprazole * Woman who is pregnant or breastfeeding * ECG: QTc \> 475 msec * Platelets ≤ 75,000/μL * Hemoglobin ≤ 9 g/dL * Neutrophils ≤ 1.0x10\^3/μL * Aspartate or alanine transaminase (AST or ALT) \> 3xULN * Serum creatinine ≥ 2 mg/dL * Patient weighed \< 15 kg * Patient who participated in any other clinical trial within 4 weeks * Patient determined to require the administration of the prohibited medications during the study period * Patient with other conditions determined by the investigator to be inappropriate for this clinical study