During an eight-year period, 17 patients ranging in age from 7 months to 81 years were found to have arteriovenous malformations (AVMs) of the gastrointestinal tract complicated by major hemorrhage. After review of these cases, a clinical classification of AVMs was developed, based on angiographic characteristics, localization, age of the patient, and family history. Type 1 AVMs were solitary, localized lesions within the right side of the colon. They occurred in seven patients 55 years of age or older. None were palpable or visible at operation. Type 2 AVMs occurred in seven patients. They were larger and occasionally visible, most commonly in the small intestine, and probably of congenital origin. Symptoms all began before 50 years of age. Type 3 AVMs were punctate angiomas causing gastrointestinal hemorrhage. They occurred in three patients with the classical findings of hereditary hemorrhagic telangiectasia. The diagnosis of most AVMs can be determined preoperatively only by selective angiographic studies.