The relationship between medical illness and depression is critically reviewed. Evidence for an association exists but is based on relatively few studies of adequate research design. Data suggest the period prevalence of depressive syndrome in medically ill patients is around 18% for the severely medically ill. An etiologic relationship has not established. The incidence of diseases in the medical subspecialities of endocrinology, neurology, cardiology, gastroenterology, and rheumatology appears to be increased in patients with depression. A causal relationship has been suggested in the first four.