Fetus in fetu (FIF) is a rare cause of abdominal mass in children. One of the malformed monozygotic diamniotic twins is located in the body of other twin. It is differentiated from teratoma by the presence of vertebral organization with limb buds and other organ systems. Diagnosis is based on radiologic findings. Surgical excision is the treatment of choice, leading to the complete removal of the mass. To our knowledge, less than 200 cases have been described in the literature. Herein, we report 2 cases of FIF, a newborn who was diagnosed antenatally and a three-and-half-year-old boy diagnosed with mediastinal FIF after admission for recurrent respiratory tract infections. (C) 2011 Elsevier Inc. All rights reserved.