The frequency of obesity, insulin resistance, type 2 diabetes mellitus and other components of metabolic syndrome appear to be significantly elevated in some psychiatric patients. This is a notable example of genetic/environment interaction. Considering the genetic contribution, evidence of insulin resistance in persons with schizophrenia was reported in the pre-pharmacological era. High insulin, glucose, and cortisol levels are observed in first episode psychosis. The frequency of type 2 diabetes mellitus is significantly increased in persons with schizophrenia and bipolar disorder and in their first-degree relatives. Finally, a link exists between schizophrenia and enzymes involved in glycolysis and between antipsychotic drug-induced weight gain and serotonin receptor polymorphism. Important environmental factors are poor dietary habits, smoking, lack of physical exercise, and drug treatment, mostly with antipsychotic drugs (APDs) and perhaps with mood stabilizers. The APDs probably induce metabolic dysfunction by producing sudden appetite increase and weight gain in predisposed subjects. However, direct drug effects on glucose and lipid metabolism independent from body weight change have been proposed. Excessive weight gain is mainly observed with clozapine, olanzapine, chlorpromazine, and thioridazine and is less consistently noted with risperidone or quetiapine. Two recently introduced APDs, ziprasidone and aripiprazole, display a neutral effect on weight and metabolism. Subjects at high risk must be identified early during APD treatment so that provide lifestyle counseling and pharmacological assistance can be provided. The immediate research agenda for the APDs is to improve the animal models of drug-induced metabolic dysfunction; to clarify mechanisms other than weight gain and appetite stimulation; and to test pharmacological agents in randomized, double-blind studies to prevent or reverse metabolic syndrome in selected patients.