Computerized register linkage and a questionnaire survey showed that the number of mentally retarded admitted to psychiatric hospitals in Denmark is rapidly increasing. This development is highly deplorable. Diagnosis and treatment are frequently complicated by the nature of the mental handicap and, especially, the often diverging clinical appearance of the psychiatric disorders in these persons. Mentally retarded with a psychiatric disorder should, in general, be treated in special institutions or outpatient clinics, by specially trained social psychiatrists and staffs.