The term "fetus-in-fetu" refers to a monozygotic, diamniotic twin that is incorporated into the body of another sibling early in embryonic development. A fetus in fetu is a differentiated mass that, in contrast to a teratoma, has a discrete spinal column and well developed internal organs. Most cases of fetus-in-fetu are discovered as a retroperitoneal mass in the first year of life, and the condition is unusual in adulthood. We describe a case of fetus-in-fetu incidentally discovered in a 39-year-old man who complained of mild epigastralgia. Abdominal ultrasound revealed a mass at the right upper quadrant of the abdomen. Fetus-in-fetu was diagnosed by computed tomography as an incidental finding, which was not clearly related to his abdominal discomfort. The fetus-in-fetu was treated conservatively. His radiologic as well as clinical condition remained unchanged during 4 years of follow-up.