This article reviews the epidemiology of weight gain and diabetes mellitus in general and in patients with severe mental illness in particular. Body mass index is defined, and possible predictors for weight gain in patients receiving antipsychotic medications are also enumerated. Information on risk of association with type 2 diabetes mellitus is described, as well as information on dyslipidemias within the rubric of the metabolic syndrome. Recent consensus panel reports and their recommendations for ongoing patient monitoring are reviewed. The issue of switching antipsychotic medication in the context of a developing metabolic disorder is discussed with regard to appropriately balancing risk and benefits. Collaborative treatment between a psychiatrist and an endocrinologist is encouraged. The primary care physician may be required to fulfill both roles.