Diagnostic sigmoidoscopy and barium enema examinations (n = 214) or colonoscopy (n = 52) were performed for acute lower gastrointestinal (GI) bleeding. The cause of bleeding was detected in 76% (203 of 266) of the primary examinations, and the cause remained unclear after subsequent examinations in 17% of the cases. Hemorrhoids were the cause of bleeding in 28% (56 of 203) of the cases, colonic diverticular disease in 19% (39 of 203), adenomatous polyps in 11% (23 of 203), and colorectal cancer in 10% (20 of 203). The sources of bleeding did not differ significantly by sex. An anorectal site of bleeding was most often detected in patients less than 50 years of age (p < 0.0001) and a left colonic site in those more than 70 years (p < 0.0001). Hemorrhoids significantly more often caused rectal bleeding among the youngest patients (p < 0.0001) and colonic diverticular disease among the oldest patients (p < 0.0001) and colonic diverticular disease among the oldest patients (p < 0.0001) than among the others. Of the 266 patients, 19% have been operated on for the bleeding lesion. The mortality related to lower GI bleeding was 4% (11 of 266). The mean age of the patients who died was higher than that of those who survived (p < 0.05).