This article reports the findings of a study investigating rates and types of comorbid mental disorder evident in adolescents and young adults with autism. A sample of 84 young people (M = 19.5 years, SD = 4.6) with "Diagnostic and Statistical Manual of Mental Disorders," 4th Edition, Text Revision (DSM-IV-TR; American Psychiatric Association, 2000) autistic disorder participated in the study. Comorbid mental disorder was assessed through a semistructured comprehensive clinical interview with all participants and caregivers and supplemented by quantitative questionnaires. The results revealed that 42% of the young people with autism in the sample had an additional comorbid mental disorder, a rate 2 to 4 times that found among typically developing young people. All diagnoses were made according to DSM-IV-TR and "International Classification of Diseases," 10th Edition criteria (ICD-10; World Health Organization, 1992). A range of mood (17%), anxiety (12%), adjustment (8%), and disruptive behavior (12%) disorders were diagnosed. (Contains 3 tables.)