The relationship between schizophrenia and mental retardation is examined. Historical associations between symptoms of the two disorders are reviewed, and a 3-year study of the incidence (14%) of mental illness in 798 retarded individuals in a community based program is described. Information on the etiological, developmental, and phenomenological factors involved in classifying psychotic reactions in the mentally retarded is summarized. Treatment with thiothixene (Navane) and thioridazine (Mellaril) on 61 retarded and non-retarded schizophrenics in an inpatient psychiatric hospital is reported to have produced significant improvement in social adjustment, attentiveness, approachability, and behavior. The thiothixene treated group required less time to show therapeutic improvement. The author asserts that schizophrenia in retarded persons is a distinct, treatable condition, and that there is increased interest in the psychiatric aspects of retardation. (CL)