Patients with severe mental illnesses such as bipolar disorder and schizophrenia have significantly increased rates of early mortality compared with the general population. Increased mortality in the mentally ill is often due to medical comorbidities associated with overweight or obesity. Weight gain can be associated with the mental disorder itself or with the pharmacotherapy used to treat the mental disorder. Effective weight-neutral and weight-reducing medical agents are available, and the use of an appropriate pharmacotherapy regimen with behavior modification and patient education may help manage body weight in patients with serious mental illnesses.