Few studies assess psychiatric disorders in representative samples of individuals with autism and ID. Symptoms of autism and psychiatric disorders have been confounded. PAC, a conceptually analysed and validated screening instrument, was used. Aims: Assess prevalence of psychiatric disorders in individuals with intellectual disability only ("ID-only") and with combination of autism and ID (autism). Sixty-two (autism) and 132 (ID-only) participants were screened for psychiatric disorders with the Psychopathology in Autism Checklist (PAC); included general adjustment problems ("GAP"), and severe adjustment problems ("SGAP") in one county in Norway. Psychosis, depression, anxiety, and OCD were addressed. Both SGAP and a high psychiatric disorder score were required to screen a psychiatric disorder. "Diagnostic overlap" was defined as more than one psychiatric disorder concurrent with autism. Psychiatric disorders and SGAP were found to be high both in the autism (53.2%) and ID-only group (17.4%). More than 50% of the autism and approximately 20% of ID-only group had SGAP. The differences were significant. The autism-psychiatric disorder interaction was significant. The largest differences between the prevalence in the autism and the ID-only group were shown in individuals with anxiety. The majority of the individuals in both study groups were afflicted with more than one psychiatric disorder. About 60% were found to have more than one disorder. The individuals with more severe psychiatric symptoms had higher degrees of diagnostic overlap. Having an intellectual disability seem to imply high risk for developing adjustment problems, and it seems especially difficult for individuals with autism to master every-day challenges. (Contains 8 tables and 1 figure.)