An ovarian mucinous cystadenocarcinoma admixed with a component of yolk sac tumor was encountered in a 71-year-old woman, the first such tumor to be reported. The preoperative serum level of alpha-fetoprotein was elevated (55.6 ng/mL). A right ovarian cystic tumor, 8 cm in maximal dimension, was removed and found to have a solid component. Microscopically, the cystic lesion was a mucinous cystadenocarcinoma, whereas the solid portion was typical yolk sac tumor with microcystic, reticular, and labyrinthine patterns, Schiller-Duval bodies, scattered giant cells with bizarre nuclei, and intracellular and extracellular hyaline globules. There was no evidence of other germ cell tumor components. Yolk sac tumor showed positive reactions for alpha-fetoprotein, carcinoembryonic antigen, and alpha-1-antitrypsin but was negative for CA125, CA19-9, and human chorionic gonadotrophin. The patient died of recurrent tumor 6 months postoperatively without response to combination chemotherapy.