We evaluated 67 patients with acute bleeding of the lower gastrointestinal tract, for diagnostic accuracy of colonoscopy, and scintigraphy. Fifty-nine patients were stable after initial resuscitation and underwent colonoscopy. The source of hemorrhage was identified in 30 patients (50.8%). Tc-labeled red blood cells scintigram was undergone in 23 patients with a sensitivity of 43.4%. The identification of the bleeding source reached 75.4% when colonoscopy was used in combination with scintigraphy. Eleven patients with lower gastrointestinal bleeding requiring transfusion of 5 units of red blood cells or more had a diagnostic exploratory laparotomy, and the diagnosis was ascertained during operation in nine. The postoperative mortality rate was 18.1%. We conclude that in patients with active gastrointestinal bleeding, colonoscopy in combination with scintigraphy detect at a higher rate the cause and the site of bleeding and possibly improve the prognosis.