Gastrointestinal bleeding is the most frequent form of bleeding after epistaxis in patients with hereditary hemorrhagic telangiectasia. As a part of an epidemiologic study, gastrointestinal telangiectases could be endoscopically demonstrated in 28 patients with hereditary hemorrhagic telangiectasia, most frequently in the upper gastrointestinal tract and predominantly in the stomach and the duodenum. The typical endoscopic finding was nodular angiomas that did not differ, with regard to form and size, from external telangiectases. However, in 15 patients some of the gastrointestinal telangiectases were surrounded by an anemic halo. A significant difference was found in the age at onset of epistaxis (median 11 yr) and of gastrointestinal bleeding (median 55.5 yr). There was no intrafamilial or interfamilial variation as to heredity and clinical manifestations. The blood group distribution in patients with hereditary hemorrhagic telangiectasia and gastrointestinal telangiectases did not differ from that of other patients with hereditary hemorrhagic telangiectasia, whereas there was a significantly higher frequency of blood group O among patients with hereditary hemorrhagic telangiectasia than among the background population.