A retrospective review of two years' experience with radionuclide screening to detect lower gastrointestinal bleeding sites was conducted at New York's Montefiore Medical Center. Of 82 studies performed in 63 patients, 13 identified active bleeding sites. Only three of eight angiograms obtained in these 13 patients were positive. Thirteen contrast angiograms were performed in the group of 50 patients with negative radionuclide studies of which ten were negative and one was equivocal. The results of this study suggest that the Tc-99m sulfur colloid study for active lower gastrointestinal (GI) bleeding is an effective screening procedure. Positive studies help determine which vessel to catheterize selectively if an angiogram is to be performed. If vascular ectasis is still suspected following a negative radionuclide study, contrast angiography can be more efficaciously performed on a nonemergent basis.