Preliminary experience with technetium-99m sulfur colloid scintigraphy in 43 patients with lower gastrointestinal bleeding is described. Within minutes of the intravenous introduction of the radiopharmaceutical, a fraction of the injected activity extravasates at the bleeding site and is eliminated from the circulation. Because of rapid clearance of the radiopharmaceutical from the vascular pool by the reticuloendothelial system, a contrast develops between the site of bleeding and surrounding background. Based on animal experiments, bleeding rates as low as 0.05-0.1 ml/min can be detected with this technique. The sensitivity of this technique in the detection of the site of hemorrhage is significantly higher than arteriography or other techniques that use radioactive blood pool indicators. In this study all patients underwent both scintigraphy and arteriography within several hours. In 20 patients with negative scintigrams, arteriography showed no evidence of bleeding. In the other 23 patients with scintigraphic evidence of hemorrhage, arteriograms were positive for bleeding only in 10. In 19 of the 23 patients, a cause for bleeding was eventually established by other means. This technique appears to offer a simple, practical, and reliable approach to the evaluation of patients with lower gastrointestinal bleeding.