A horseshoe kidney was incidentally diagnosed while performing an abdominal scan using autologous human red blood cells labeled with Tc-99m (Tc-99m RBC) on a patient with suspected gastrointestinal bleeding. This patient repeatedly tested positive on occult blood stool examinations. In the first scintigram, an increase in well-defined activity was found on the right side under the liver. This could have been caused either by bleeding of the gastric antrum or the duodenal bulb. A second abdominal scan with Tc-99m RBC revealed faint radionuclide accumulation in the shape of a horseshoe with symmetrical, well-defined activity. However, the well-defined activity decreased with time on both images, and on this basis a horseshoe kidney was diagnosed.