The authors prospectively evaluated the usefulness of abdominal radionuclide scintigraphy using 99mTc-labeled red cells as a means of monitoring for intermittent gastrointestinal bleeding over a 24-hour period in both control and actively bleeding populations. Of 32 patients with documented hemorrhage, 29 had positive scintiscans (sensitivity, 91%; 9% false negatives). Of 18 nonbleeding patients, 17 had negative scintiscans (specificity, 95%; 5% false positives). 12 of 29 patients bled from 6 to 24 hours after the study was begun. Scintiscans were positive in patients with transfusion requirements of ≥500 ml/24 hr. The authors conclude that abdominal scintigraphy with 99mTc-labeled red cells is an effective method of detecting gastrointestinal bleeding.