The diagnosis of a bleeding Meckel's diverticulum containing ectopic gastric mucosa by 99m-Tc pertechnetate imaging is reported to have a diagnostic accuracy of 90% for patients in the pediatric age group. The usefulness of Meckel's scanning in adults is, however, less certain. We present two patients who illustrate some of the difficulties inherent in Meckel's scanning in adults; one had a false-negative scan despite the presence of ectopic gastric mucosa and the other had a false-positive scan, both confirmed by laparotomy. The available literature indicates that false-positive and false-negative scans occur relatively frequently in adults, but no comprehensive review on this subject exists. Based on our findings, it is suggested that Meckel's scanning be supplemented with small bowel infusion or arteriography or both to improve preoperative diagnosis in adult patients in whom a diverticulum is suspected.