The Lundby Study is a prospective cohort study, which has followed a Swedish unselected community sample between 1 July 1947 and 1 July 1997. The aim was to study the risks of mental morbidity and different DSM-IV disorders in subjects with intellectual disability (ID) in the Lundby cohort between 1 July 1947 to 30 June 1997. The diagnosis of ID was re-evaluated according to DSM-IV in subjects who had been considered to have ID between 1947 and 1997. Multiple sources of information were used to obtain best estimate consensus diagnoses of mental disorders. The relative risk of mental disorder was 1.34 in subjects with ID as compared with the reference group. Dual diagnosis was more prevalent in mild ID than in moderate ID. No subject with severe ID was diagnosed with mental disorder. The cumulative incidence of any mental disorder in subjects with ID was 44%. The most common DSM-IV diagnoses were: Mood Disorders (11.5%), Anxiety Disorders (11.5%), Schizophrenia and Other Psychotic Disorders (8%), Mental Disorder NOS Due to a General Medical Condition (8%), Dementia (3.8%) and Alcohol Abuse (1.9%). Mental disorders were more common in subjects with ID than in the reference group.