Adverse effects associated with nonsteroidal antiinflammatory drug treatment are reported more commonly to regulatory authorities than the adverse effects of any other form of treatment. Epidemiologic evidence in general suggests a doubling or quadrupling of the risk of ulcer complications or death in recipients of such treatment. The risk appears to be related to increasing age, but no other associated factor has yet been identified. It is uncertain whether individuals with preexisting ulcer are at special risk or whether treatment predisposed equally to ulcer and ulcer complications, and there is no clear evidence that the nature of the disability leading to nonsteroidal antiinflammatory drug treatment influences the risk of gastrointestinal complications.