The clinical consequences of lower gastrointestinal bleeding range from trivial to life-threatening. Nuclear medicine imaging techniques allow identification of those patients who are actively bleeding. The demonstration of active bleeding not only provides important prognostic information but also allows for a coordinated imaging approach using angiography and aids localization of the bleeding site. (99m)Tc-labeled erythrocytes and (99m)Tc sulfur colloid are 2 commonly used techniques to detect active bleeding. Each has its respective advantages and disadvantages, but the medical literature indicates that both tests are useful. More prolonged or delayed imaging is possible using (99m)Tc-labeled erythrocytes but care is required to prevent misinterpretation of the bleeding location because of a higher likelihood of radiotracer movement through the bowel away from the bleeding site. These forms of scintigraphy may be helpful in risk-stratifying patients and planning radiological and surgical interventions. Careful selection of patients to include those who have a high likelihood of active bleeding greatly increases the clinical utility of these tests. In addition, (99m)Tc pertechnetate imaging may be diagnostic of ectopic gastric mucosa in a Meckel's diverticulum as a potential source of bleeding. Patients also should be carefully selected for this test, based on age and exclusion of other causes of bleeding.