Using a modified in vivo Tc-99m red cell labeling technique, gastrointestinal (GI) bleeding scintigraphy was performed in 100 patients with GI bleeding. Sixty-two patients with melena or brigtt red blood per rectum had positive scintiscans. In comparison to results of angiography, endoscopy, surgery and contrast radiography, radionuclide scintigraphy correctly located the site of bleeding in 83% of patients. The procedure could be performed over a 24 hr period which increased the sensitivity of the test since 85% of the scintiscans were positive at one hr or greater after the onset of imaging. The procedure was more sensitive than angiography in detecting sources of GI bleeding. We conclude that GI bleeding scintigraphy 99mTc-red cells is an accurate and effective method to detect upper and lower GI bleeding in patients with acute or intermittent gastrointestinal bleeding.