A 12-year-old girl was admitted to the hospital for evaluation of an abdominal mass. A preoperative computed tomography scan showed a large tumor in the pelvis. The serum alpha-fetoprotein level was 2,170,000 ng/ml. A 3000-g left ovarian neoplasm was resected. It was encapsulated and showed focal microcystic degeneration, necrosis, and hemorrhage. Microscopically, it was composed of gland-like spaces containing mucin-positive material and surrounded by scant fibrovascular tissue. The epithelial cells were low columnar with immature oval, basophilic nuclei. Immunoperoxidase staining for alpha-fetoprotein and alpha1-antitrypsin were positive. Enzyme histochemistry was negative for alkaline phosphatase and positive for alpha-naphthyl acetate esterase. Electron microscopy, including freeze-fracture analysis, showed desmosomes and tight junctions. No gap junctions were identified. Actin filaments, glycogen, and microvilli were abundant. This is the first case of an ovarian endodermal sinus tumor with exclusive enteric differentiation.