The aim of this study was to investigate the prevalence of mental disorders in a Norwegian sample of adults with intellectual disability (ID) using the "Psychopathology Checklists for Adults With Intellectual Disability" (P-AID; Hove & Havik, 2008), a screening instrument adopting diagnostic criteria from the "Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Intellectual Disability/Mental Retardation" (DC-LD; Royal College of Psychiatrists, 2001). The P-AID checklists, comprising 10 psychiatric diagnoses and 8 types of problem behaviors, were completed by staff at community-based homes for adults with ID in Western Norway. One in three adults with ID showed indices of a mental disorder. Problem behavior was the most prevalent single diagnosis next to anxiety and affective disorder, and there was a high frequency of comorbidity among the disorders. The findings extend previous reports on mental disorders and comorbidity and support the use of DC-LD in epidemiological research. (Contains 6 tables.)