OBJECTIVE This study aimed to establish the prevalence of diabetes mellitus (DM) and impaired glucose tolerance in patients with schizophrenia and to elucidate some of the factors associated with hyperglycemia. METHODS We studied a sample of 194 patients with schizophrenia. We determined fasting blood glucose and insulin levels at the start of the testing period; patients were given an oral glucose tolerance test after an overnight fast. RESULTS The overall prevalence of diabetes and impaired glucose tolerance in these patients was 16.0% and 30.9%, respectively. These rates were higher than those reported in the general population. Patients with disordered glucose homeostasis were significantly older (P = 0.005, Kruskal-Wallis test). There was no significant effect of sex or ethnicity on diabetes. CONCLUSIONS Our findings suggest that patients with schizophrenia are more vulnerable to developing DM. We caution clinicians to be mindful of the increased risk and to be vigilant for such a development.