Retroperitoneal fetus in fetu in a 65-year-old man

Fetus in fetu (FIF) is a rare congenital anomaly in which a malformed fetus grows within the body of its twin. It was first described in the late 18th century and has an incidence of 1:500,000 live births. The diagnosis is established in infancy or young adults in most cases. The oldest patient reported so far was a 47-year-old patient. We described the case of a 65-year-old patient with FIF, the oldest reported in the literature. Our patient meets all criteria for the diagnosis of FIF, with the presence of a limb in advanced formation inside the lesion. The treatment was surgical excision. FIF must be considered in the differential diagnosis of an abdominal mass, typically recognized in infancy. Symptoms arise from the mass effect and treatment is surgical resection due to the potential for malignancy.