The clinical and pathologic features of 15 examples of a hitherto undescribed germ cell tumor of the ovary are delineated. This tumor resembles the embryonal carcinoma of the adult testis and may be distinguished from the endodermal sinus tumor on the basis of its histologic and immunohistochemical characteristics. An indirect immunoperoxidase method for the localization of human chorionic gonadotropin (HCG) and alpha-fetoprotein (AFP) was done on formalin-fixed paraffin-embedded tissue from 10 neoplasms; HCG was present in all 10 neoplasms, and AFP was found in seven. HCG was indentified only in cells resembling syncytiotrophoblast, whereas AFP was present only in mononuclear embryonal cells, indicating that the two protein antigens were localized in different cell types. Abnormal hormonal manifestations, consisting of precocious puberty, irregular bleeding, amenorrhea, or hirsutism, were present in nine (60%) of the patients. The actuarial survival for the entire group was 39%; for those with stage I tumors, 50%. We are proposing the term "embryonal carcinoma" for this neoplasm in order to distinuish it from the more common endodermal sinus tumor of the ovary and to emphasize the histologic similarity to embryonal carcinoma of the adult testis.