Dysgerminoma has traditionally been considered an end-stage neoplasm without potential for further differentiation. Although there have been several reports of transformation of testicular seminoma to yolk sac tumor, a similar event has not been previously reported in dysgerminoma of the ovary. Three cases of ovarian germ cell tumor (two pure dysgerminomas and one mixed germ cell tumor with dysgerminoma and yolk sac components) that revealed histologic changes compatible with early transformation to yolk sac tumor are described. In general, the areas of transformation were located at the periphery of the tumor lobules which otherwise had features of typical dysgerminoma. They were characterized by the presence of microcysts and small glandular structures, which though not readily identified on H&E became more evident with stains for keratins, alpha-fetoprotein, and blood group-related antigen. The small size and focal nature of change, and the apparent transition favor the interpretation that this change represents transformation rather than admixture of two germ cell components. The relationship of dysgerminoma to the solid variant of yolk sac tumor is discussed and an alternate histogenetic scheme in which dysgerminoma represents the stage of earliest differentiation from which other non dysgerminomatous tumors may arise is presented. Although previously proposed for testicular germ cell neoplasia, this scheme has not yet been applied to their ovarian counterparts.