Seventy-six patients clinically suspected of having lower gastrointestinal bleeding were studied by scintigraphy utilizing red blood cells labeled in vitro with technetium Tc 99m. Sixteen patients required emergency surgery; bleeding was accurately localized in 15 (94%). One patient (6%) had a normal scan. A 20-month mean follow-up of the 16 patients showed no recurrent bleeding. Of 60 patients not requiring emergency surgery, bleeding was localized in 11, but the bleeding ceased. Forty-nine of the 60 patients had normal scans and had no further hemorrhaging during hospitalization. A 21-month mean follow-up of 38 of the 49 patients showed no further bleeding episodes or surgical procedures in 29 patients; however, eight patients required surgical procedures, including seven for gastrointestinal malignancies. Scanning of red blood cells labeled in vitro with 99mTc is accurate and efficacious in localization of bleeding sites that require emergency surgery for lower gastrointestinal hemorrhage.