BACKGROUND: Psychiatric comorbidity of autism spectrum disorder (ASD) has not been well examined. METHODS: Mood disorders in 44 consecutive outpatients with high-functioning ASD were examined at a university hospital according to DSM-IV. Inclusion criteria were an IQ of 70 or higher on the Wechsler Intelligence Scale and age of 12 years or over. RESULTS: Sixteen patients (36.4%) were diagnosed with mood disorder. Of these 16 patients, four were diagnosed as having major depressive disorder, two patients as bipolar I disorder, six patients as bipolar II disorder, and four patients as bipolar disorder not otherwise specified. Bipolar disorder accounted for 75% of cases. Twelve patients had Asperger disorder and four patients had pervasive developmental disorder not otherwise specified. None of the patients had autistic disorder. LIMITATIONS: The sample size was small. We could not use Autism Diagnostic Interview - Revised. Referral bias could not be avoided in this study. CONCLUSIONS: The major comorbid mood disorder in patients with high-functioning ASD is bipolar disorder and not major depressive disorder. The autistic spectrum may share common vulnerability genes with the bipolar spectrum.