Sixty-four patients with symptomatic hereditary hemorrhagic telangiectasia were retrospectively studied in order to determine the true incidence of clinical manifestations in this disease. This select group had a significantly higher incidence of gastrointestinal hemorrhage and pulmonary arteriovenous fistula formation than has been previously reported. Data are presented regarding the course and severity of nasal and gastrointestinal hemorrhage, the use of endoscopy for diagnosis, the incidence of associated neurological, cardiac, and hepatic disease, and mortality.