A retrospective study was conducted using 36 patients with gastrointestinal bleeding in whom the diagnosis was not directly apparent from first line diagnostic procedures. Final diagnosis was established by surgery, endoscopy, or postmortem examination in 20 patients. Scintigraphic examination with Tc-99m or In-111 labeled red blood cells yielded 24 positive and 18 negative results. Nine out of 13 positive scans (verified by other diagnostic procedures) accurately identified the site of bleeding. This was considered to be a satisfactory result in this group of difficult to diagnose patients. The lowest success rate was observed in patients taking drugs that interfered with coagulation, or in patients prone to diffuse blood loss because of coagulopathy. Late scans did not offer additional information and the use of In-111 for this purpose was not thought to be of benefit. Although the technique is rather noninvasive and simple, its application should be restricted to selected patients and its interpretation related to the results of other investigations.