Angiodysplasia is an important vascular lesion of the gut and a source of significant morbidity from bleeding. This lesion is probably responsible for approximately 6.0% of cases of lower gastrointestinal (GI) bleeding and 1.2-8.0% of cases of hemorrhage from the upper GI tract. Small bowel angiodysplasia accounts for 30-40% of cases of GI bleeding of obscure origin and represents the single most common cause for hemorrhage in this subset of patients. Lesions in the large bowel occur most often in the right colon. Their cause is unknown but most are probably acquired and the result of a degenerative process associated with aging. The incidence of colonic angiodysplasia among strictly asymptomatic individuals has never been determined and the natural history for these lesions is incompletely understood. Angiodysplasia in the upper GI tract occurs most often in the stomach and duodenum. When affected patients have been evaluated by colonoscopy concomitant lesions have been diagnosed in one-third of instances. Angiodysplasia has been purported to occur with higher frequency in patients with renal failure, von Willebrand's disease, aortic stenosis, cirrhosis, and pulmonary disease. Not all of these associations have been subjected to critical analysis, but available evidence does not support a strong relationship in most instances. Patients with bleeding angiodysplasia are occasionally treated with hormones or, more often, by endoscopic methods. Uncontrolled case studies have reported reduction or cessation of bleeding in subjects managed with conjugated estrogens. However, prospective randomized controlled trials assessing the efficacy of hormonal therapy are limited, and results from two trials conflict. Safety profiles for the endoscopic methods are acceptable, and reported efficacies are high, although not all methods have been extensively evaluated specifically for the treatment of angiodysplasia. Perforation of the right colon is a potential problem, especially for monopolar electrocoagulation and lasers.