BACKGROUND A number of studies have examined the prevalence of diabetes mellitus and impaired glucose tolerance in general populations and in those with schizophrenia and other forms of serious mental illness. OBJECTIVE To establish whether it is possible to describe accurately comparative rates of diabetes mellitus and impaired glucose tolerance in populations of people with schizophrenia and those without mental illness. METHODS Review of current literature. RESULTS Research published in the pre-neuroleptic era suggested that people with severe mental illness were at increased risk of developing glycaemic abnormalities. Recent studies appear to confirm that the prevalence of diabetes and impaired glucose tolerance may be higher in people with schizophrenia than in the general population, and suggest that patients with schizophrenia have impaired glucose tolerance even before they begin treatment. CONCLUSIONS Schizophrenia may be a significant and independent risk factor for both diabetes and impaired glucose tolerance. Current data preclude precise estimates of the prevalence of these conditions among people with schizophrenia.