This study describes the social competence and maladaptive behavior of mentally retarded individuals with psychiatric impairments and examines the validity of five major psychiatric diagnoses as applied to the mentally retarded population: schizophrenia, personality disorders, autism, adjustment disorders, and conduct disorders. The adaptive and maladaptive behavior of individuals with a dual diagnosis (N=3,975) have been compared with the behavior of control samples who were selected by a pair-wise matching procedure on age, sex, IQ, and type of residence (i.e., own home or independent living, community care facilities, and state institutions). Among the dual diagnosis population, schizophrenic adults and autistic children have shown disturbances in many areas of adaptive and maladaptive behavior in comparison to their nonpsychiatric counterparts. Conduct disturbances, personality disorders, and adjustment disorders have shown significantly higher maladaptive behavior than their non- psychiatric counterparts. The results indicate a strong need for refined diagnostic techniques for identifying personality disorders and adjustment disorders in the mentally retarded population.