302 mentally retarded adults, sampled by epidemiological criteria, were examined with regard to handicaps, behaviour, skills and psychopathology by use of the MRC HBS-schedule and a list of psychiatric items. Based on research criteria, a computerized psychiatric diagnosis was made on a hierarchial scale. A psychiatric disorder was diagnosed in 85 (27.1%), which is a smaller prevalence rate than found in other studies. Next to behaviour disorder (10.9%), psychosis of uncertain type (5%) was the most common disorder. Dementia and early childhood autism were found equally often (3.6% each). Neurosis was seldom (2%), while schizophrenia (1.3%) and affective disorder (1.7%) occurred at about the same rates as found in similar investigations. No cases of alcohol or drug abuse were found.