Type 2 diabetes mellitus and obesity have reached epidemic proportions in many developing and developed nations, leading to talk of the "twin epidemics." The latest projections from the International Diabetes Federation suggest that 190 million people worldwide currently have type 2 diabetes. In addition, > or = 300 million people worldwide have impaired glucose tolerance (IGT). These statistics represent an epidemic of major proportions--possibly the largest epidemic in human history--in terms of glucose intolerance and cardiovascular disease (CVD) risk because individuals with IGT are at substantially higher risk for diabetes and CVD than are members of the general population. Along with IGT, the metabolic syndrome comprises other major CVD risk factors, including insulin resistance, central obesity, and dyslipidemia; insulin resistance has been implicated as the single most common cause of the syndrome. Although the exact prevalence of the metabolic syndrome is unknown, the syndrome is widespread among adults in developed nations, becoming more prevalent with age. Epidemiologic data suggest that in patients with schizophrenia or affective disorders, both diabetes and obesity are 1.5 to 2.0 times more prevalent than in the general population. Furthermore, because adverse effects of certain therapies for human immunodeficiency virus (HIV) infection and psychiatric disorders increase the risk for developing diabetes, obesity, and the metabolic syndrome, such therapies should be carefully chosen, particularly considering CVD risk. Appropriate therapy may be determined via screening of patients for levels of fasting blood glucose and lipids, as well as other CVD risk factors, before initiating use of second-generation antipsychotic agents or highly active antiretroviral therapy.