Objective: To investigate the relationship between behavioural disturbance and psychiatric diagnosis in male mentally handicapped adults resident in a long stay unit. Method: The case notes of all 144 residents of St. Raphaels were reviewed. Diagnoses were classified according to the International Classification of Diseases Ninth Revision (ICD-9) and grouped into one of four categories Infantile Autism, Functional Psychoses, Personality/Behavioural Disorder and Emotional Disorders. Carers (Senior Nursing Staff) were interviewed using the Adaptive Behaviour Scale Part 2 (ABS Part 2) as a measure of behavioural disturbance in the preceding year for the 144 residents. Results: 56% of residents had significant psychiatric disorder. Those with a psychiatric diagnosis showed increased behavioural disturbance across a wide range of measures (p<0.001) with Hyperactivity, Self Abuse and Violence showing greatest discriminative power. However there was not a relationship between diagnostic category and pattern of scoring on the ABS Part 2. There was a significant relationship between Degree of Mental Handicap and Psychiatric Diagnostic Category (p<0.001). Neither psychiatric morbidity nor behavioural disturbance was associated with epilepsy, age or duration of stay. Conclusions: Mentally handicapped adults in residential care show high rates of psychiatric disturbance. Those with psychiatric diagnoses exhibit a wide range of disturbed behaviours but similar patterns of maladaptive behaviour occur across the spectrum of psychiatric disorders. These findings refer to a male residential population and cannot be generalised to all mentally handicapped.