Meckel's diverticulum, an omphalomesenteric duct remnant, is a congenital outpouching usually located in the distal 100 cm of the ileum. Such diverticula are present in 1-3% of the general population. Of these, 30-40% are asymptomatic. Complications of Meckel's diverticulum include hemorrhage, intestinal obstruction, diverticulitis, umbilical discharge, perforation and peritonitis. Bleeding is the most common complication, resulting from ulceration of the ileal mucosa adjacent to ectopic gastric mucosa located within the diverticulum. (However, in the patients with a Meckel's diverticulum but without ectopic gastric mucosa, bleeding does not usually occur.) This bleeding is often painless and is usually encountered in children and young adults. Meckel's diverticulum accounts for nearly 50% of all lower gastrointestinal bleeding in children. Technetium-99m pertechnetate is normally taken by the ectopic gastric mucosa, providing the basis for the Meckel scan. Since only 60% of Meckel's diverticula contain ectopic gastric mucosa, false negative results occur. If the scan is positive, increased sensitivity can be achieved by repeating the scan after a short course of an H₂-RA: the H₂-RA releases acid secretion by the ectopic parietal cells in the Meckel's diverticulum and may thereby convert a positive into a negative scan.