The prevalence of psychiatric illness was studied in 150 epileptic mentally handicapped people (both hospital in-patients and living in the community) and a matched group of 150 non-epileptic controls. The Profile of Abilities and Adjustment (PAA) scale was used for the initial screening of psychiatric illness. Mildly to moderately handicapped individuals who had good communication skills and scored positively on the PAA schedule for psychiatric illness were interviewed using the PSE interview schedule. Severely mentally handicapped individuals who scored positively on the PAA's psychiatric illness subscale were observed and information was gathered from their medical notes and carers. A psychiatric diagnosis was made using DSM–III–R criteria. The non-epileptic group showed significantly more psychiatric illness than the epileptic group. Psychiatric illness was diagnosed in 25% of the cohort.