Occult gastrointestinal bleeding is defined as gastrointestinal bleeding that is not visible to the patient or physician, resulting in either a positive fecal occult blood test, or iron deficiency anemia with or without a positive fecal occult blood test. A stepwise evaluation will identify the cause of bleeding in the majority of patients. Esophagogastroduodenoscopy (EGD) and colonoscopy will find the bleeding source in 48 to 71 percent of patients. In patients with recurrent bleeding, repeat EGD and colonoscopy may find missed lesions in 35 percent of those who had negative initial findings. If a cause is not found after EGD and colonoscopy have been performed, capsule endoscopy has a diagnostic yield of 61 to 74 percent. Deep enteroscopy reaches into the mid and distal small bowel to further investigate and treat lesions found during capsule endoscopy or computed tomographic enterography. Evaluation of a patient who has a positive fecal occult blood test without iron deficiency anemia should begin with colonoscopy; asymptomatic patients whose colonoscopic findings are negative do not require further study unless anemia develops. All men and postmenopausal women with iron deficiency anemia, and premenopausal women who have iron deficiency anemia that cannot be explained by heavy menses, should be evaluated for occult gastrointestinal bleeding. Physicians should not attribute a positive fecal occult blood test to low-dose aspirin or anticoagulant medications without further evaluation.