The reported prevalence of psychiatric illness among adults with intellectual disability (ID) varies widely between 10 and 39%; however, many methodological problems exist. The aims of the present study were to establish the prevalence of functional psychiatric illness among adults with ID who live in the community, in order to compare the overall rate and types of psychiatric illness between the population with ID and the general population without ID, and to establish the risk factors associated with psychiatric illness in adults with ID. The study was done in two stages. In the first part, a trained psychiatrist interviewed 101 randomly selected adults with ID and their carers using the Mini Psychiatric Assessment Schedule for adults with Developmental Disability (Mini PAS-ADD) to screen for psychiatric caseness. Out of these 101 adults, 90 had sufficient communicative abilities that made the administration of Mini PAS-ADD possible. A second trained psychiatrist interviewed 19 out of the 20 adults who were diagnosed as psychiatric cases according to the initial Mini PAS-ADD interview. This psychiatrist interviewed patients and their carers in line with the full PAS-ADD interview. The second psychiatrist was blind to the initial diagnoses made according to the Mini PAS-ADD questionnaire. A final psychiatric diagnosis was made according to International Classification of Diseases - 10th Revision (ICD-10) criteria. Some 14.4% (95% confidence interval = 7.4-21.4%) of the cohort had a psychiatric diagnosis according to ICD-10 criteria: 4.4% had schizophrenia, 2.2% depressive disorder, 2.2% generalized anxiety disorder, 4.4% phobic disorder and 1% delusional disorder. The overall rate of functional psychiatric illness (point prevalence) was similar to that found in the general population (16%). However, the rates of schizophrenic illness and phobic disorder were significantly higher in the study cohort compared with those in the general population (0.4% and 1.1%, respectively). Increasing age and the presence of physical disability were significantly associated with the occurrence of psychiatric illness. Out of the 11 remaining adults with severe ID, two (18%) had a diagnosis of a psychiatric illness (one mania and one anxiety disorder) according to the Diagnostic Assessment for the Severely Handicapped (DASH) questionnaire.