Scanning with Technetium 99m labeled autologous red blood cells was performed in 59 patients with clinical suspicion of acute and/or intermittent, chronic gastrointestinal bleeding. In 36 patients (61%), a definite site of bleeding could be demonstrated. A strong correlation with other modalities such as upper and lower gastrointestinal endoscopy, contrast angiography, and surgical exploration was found. Overall sensitivity of the procedure was 91%; specificity 100% and accuracy 93.3%. It is suggested that radionuclide scintigraphy provides a completely noninvasive, simple, and sensitive procedure which may be routinely used for the detection and localization of gastrointestinal bleeding.