Twenty-one of fifteen hundred twenty-six patients with Crohn's disease (CD) treated at The Mount Sinai Hospital between 1960 and 1986 developed severe gastrointestinal hemorrhage. There were 26 separate episodes of severe hemorrhage: 17 patients bled only once, three bled twice and one bled three times. The frequency of bleeding was significantly higher among patients with colonic involvement (17 of 929; 1.9%) than among those with small bowel disease alone (4 of 597; 0.7%) (p less than 0.001). Twelve patients required surgery on 13 occasions, which involved colon resection in all but one case. Eleven of these patients underwent surgery during their first hemorrhagic episodes, and 1 of 11 had a second operation for recurrent bleeding; the 12th patient, whose first hemorrhage had been treated medically, had surgery during a repeated episode of hemorrhage. The precise bleeding points could be located in only 2 of the 26 bleeding episodes, both at the ileocecal area. Three patients died, of whom two had not undergone surgery when they had bled a few weeks earlier. Primary bleeding episodes subsided without surgery in 10 of 21 cases, but 3 of these 10 patients (30%) rebled massively. By contrast primary excisional surgery was followed by recurrent hemorrhage in only 1 of 11 cases (9%). These differences in mortality and in recurrent bleeding rates, although not statistically significant, seem to favor removal of diseased bowel at the time of the first episode of massive hemorrhage.