Technetium-99m ((99m)Tc) pertechnetate scintigraphy in a child with acute gastrointestinal bleeding of unknown origin suggests ectopic gastric mucosa caused by Meckel's diverticulum or gastrointestinal duplication cysts. Our objective was to define the patterns of scintigraphic findings likely to be encountered in patients with ectopic gastric mucosa with illustrative cases and to review the literature. Fifty children (age 1 year to 14 years) were evaluated for ectopic gastric mucosa using (99m)Tc pertechnetate scintigraphy. Functioning ectopic gastric mucosa was detected in eight patients with Meckel's diverticula and three patients with bowel duplication. Three patients showed atypical findings on scintigraphy which were perforated appendix, calyceal stasis, and regional enteritis. Ectopic functioning gastric mucosa in Meckel's diverticulum is visualized simultaneously with the stomach, whereas in intestinal duplications tracer activity can be visualized in the dynamic sequence or before gastric tracer visualization in an irregular pattern. A variety of scintigraphic patterns can be found in patients with ectopic gastric mucosa undergoing (99m)Tc pertechnetate scintigraphy depending on the location and size of the ectopic tissue. Also, acquisition of delayed images is useful when the initial images are equivocal in children.