A 22-year-old man with burning pain in the lower abdomen and rectal bleeding had a sodium pertechnetate Tc 99m scan. The scan showed an area of uptake in the right lower quadrant which at operation was found to be a Meckel's diverticulum. The ability of ectopic gastric mucosa within a Meckel's diverticulum to concentrate sodium pertechnetate Tc 99 m allows for this noninvasive diagnosis of some types of rectal bleeding, especially when the cause has escaped detection by conventional endoscopic and barium contrast examinations.