The results of a small scale retrospective study are presented. The study was initiated to determine the incidence of diabetes in long-stay patients in two psychiatric hospitals in Northern Ireland and to attempt to define whether there was a relationship between the two disease states, schizophrenia and diabetes mellitus. There was a higher percentage of diabetics in the two institutions than was expected. It was concluded that drug therapy of the mental illness may have had a contributory effect on the subsequent development of diabetes mellitus.