Within a two-year period, 12 patients with upper gastrointestinal bleeding due to gastroduodenal vascular malformations were admitted to a specialized intensive care unit. They represented 2.1% of all admissions for upper gastrointestinal bleeding, and 3.7% of those with severe hemorrhage (greater than or equal to 2 units of blood transfused). Early endoscopy was nondiagnostic during the first bleeding episode in all nine patients with nonhereditary vascular malformations; the diagnosis was eventually made after relapsing hemorrhage by repeat endoscopy in five patients, angiography in two, and histology in another two. These nine patients accounted for 23.6% of all cases of upper gastrointestinal bleeding considered to be of unknown origin after initial work-up. The remaining three patients had Rendu-Osler-Weber disease, and the first endoscopy was diagnostic in all of them. Emergency treatment was required for 11 patients; surgery was undertaken in seven, and transendoscopic therapy (electrocoagulation or endoscopic clipping) in four. There was a 25% mortality rate. No further bleeding has occurred in eight patients after a mean follow-up period of two years. Gastroduodenal vascular malformations are a more frequent cause of upper gastrointestinal bleeding than heretofore recognized, especially in patients whose hemorrhage is deemed to be of unknown origin after an initial work-up. Endoscopy, which may need to be repeated, is the most rewarding diagnostic procedure. Awareness of their possible existence facilitates endoscopic recognition. When available, transendoscopic therapy is a good choice to stop active bleeding from such lesions.