The clinical and pathological features of eight ovarian yolk sac tumors with glandular patterns resembling those of endometrioid adenocarcinoma are described. The patients ranged in age from 11 to 34 years (mean, 22 years) and presented with abdominal pain or swelling. The serum alpha-fetoprotein (AFP) level was elevated at the time of presentation or later in all seven patients in whom it was measured. Seven tumors were unilateral, one was bilateral, and three had spread beyond the ovary. There was a contralateral streak gonad in two cases. The tumors were 6-35 cm in diameter; seven were solid and cystic, and one was a unilocular cyst with a small solid nodule in the wall. Microscopic examination revealed a prominent, and in two cases, pure endometrioid-like glandular pattern that often simulated that of an early secretory endometrium. Reticular, polyvesicular-vitelline, and hepatoid patterns of yolk sac tumor were also present in five tumors; minor teratomatous foci (squamous epithelium and cartilage) were present in one. Immunohistochemical staining revealed AFP, alpha-1-anti-trypsin (AAT), and carcinoembryonic antigen within the glandular epithelium; AFP and AAT were also present in areas showing the other patterns. Three patients died of recurrent or metastatic tumor 19-60 months postoperatively; in the remaining cases, there was a tumor-free follow-up of short duration. The endometrioid-like pattern reflects an unusual form of endodermal differentiation within yolk sac tumors that should be distinguished from endometrioid carcinoma.