BACKGROUND 99mTc red blood cell scintigraphy is a non invasive diagnostic method for low flow gastrointestinal bleeding. OBJECTIVE To assess the diagnostic yield of this method in patients admitted with gastrointestinal bleeding in whom upper gastrointestinal endoscopy was negative. METHODS The clinical records of 59 patients, aged 6 to 90 years old (35 male), with active gastrointestinal bleeding subjected to a 99mTc red blood cell scintigraphy were reviewed. All had non diagnostic upper gastrointestinal endoscopic studies, and in 20 a selective arteriography was performed. RESULTS In 40 patients the scintigraphy was positive for gastrointestinal bleeding, and in 57% of these the exam was positive during the first hour. Fifteen of the 19 patients with a negative scintigraphy did not have an active clinical bleeding at the moment of the examination. In 24 patients, a final etiological diagnosis was reached. In 93% of these patients scintigraphy correctly identified the bleeding site. In one patient with a negative scintigraphy, angiography disclosed a pseudo aneurysm of the splenic artery that was not bleeding actively. CONCLUSIONS In these patients with negative upper gastrointestinal endoscopy 99mTc red blood cell scintigraphy had a 91% sensitivity for the diagnosis of active gastrointestinal bleeding.