In most patients with upper gastrointestinal (GI) bleeding endoscopy will locate the site and cause of bleeding, and also provide an opportunity for local therapy. The cause of lower GI bleeding is often difficult to attribute, even when pathology is found by colonoscopy or barium enema. Nuclear medicine techniques can be used to identify the site of bleeding in those patients in whom the initial diagnostic procedures are negative or inconclusive. Methods using transient labelling of blood (e.g. 99Tcm-sulphur colloid) produce a high target-to-background ratio in positive cases, give quick results and localize bleeding sites accurately, but depend upon bleeding being active at the time of injection. Techniques using stable blood labelling (e.g. 99Tcm-labelled red blood cells) may be positive even with intermittent bleeding but may take several hours to produce a result and are less precise in localization. In order for these methods to become more widely accepted by physicians and surgeons, and for them to be cost-effective, patients should be carefully selected. The most useful application is in patients with recurrent or prolonged bleeding, those with inconclusive endoscopy or barium studies, and those who are high-risk surgical candidates.