Individuals with attention-deficit/hyperactivity disorder (AD/HD) and autism spectrum disorders (ASD) often display symptoms from other diagnostic categories. Exclusion criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) and the International Statistical Classification of Diseases and Related Health Problems (ICD-10) impede the use of categorical diagnoses to describe the particular problem constellation in a patient. In this study, we describe the prevalence and patterns of comorbid bipolar and psychotic disorders in 241 consecutively referred adult patients with AD/HD and/or ASD. Thirty per cent of patients with AD/HD had comorbid ASD and 38% of patients with ASD had comorbid AD/HD. Of the subjects with ASD, 7% had bipolar disorder with psychotic features, and 7.8% had schizophrenia or another psychotic disorder. The corresponding figures for the patients with AD/HD were 5.0% and 5.0%, respectively. Current diagnostic criteria have to be revised to acknowledge the comorbidity of bipolar and/or psychotic disorders in AD/HD and ASD.