In 160 cases with granulomatous colitis or ileocolitis, blood was found in the stool in 72 or 45%. Massive bleeding occurred in four patients (25%); overt bleeding in 51 (32%) and occult bleeding, repeated guaiac positive stools, in 17 (11%). Thus, clinically obvious bleeding occurred in about one patient in three. Of the four patients with massive hemorrhage one required subtotal colectomy, one right hemicolectomy and the other two were controlled by medical measures including blood transfusion. In each of the surgical cases, the massive bleeding originated in the region of the cecum. An additional patient, not in the series, required proctectomy as a life-saving measure following subtotal colectomy. Clinically obvious rectal bleeding in Crohn's colitis is approximately ten times as frequent as in regional enteritis but only one-third as frequent as in ulcerative colitis. There was no correlation between rectal bleeding and rectal involvement determined radiologically or by signoidoscopy, in the whole series but there was a strong association in the small group of Crohn's colitis with rectal involvement (86% with bleeding) and a significantly greater proportion of patients with colitis with rectal involvement and hemorrhage (P smaller than 0.02). Anemia is common in both groups but in Crohn's colitis it is almost always, 75%, associated with rectal blood loss, whereas in ileocolitis this association is much less marked, 40%, indicating other important causes of anemia when there is small bowel disease. The bleeding patient with Crohn's colitis should be managed medically initially. Uncontrollable, continuous, or massive hemorrhage may require angiography and early surgical intervention.