A 14-year-old girl with massive ovarian edema (MOE) of the right ovary and a polycystic ovary is presented. The right ovary was moderately enlarged and incompletely twisted. Histologically, it showed diffuse interstitial edema and multiple cysts, frequently surrounded by a line of luteinized cells. Such cysts are suspected to consist of highly degenerative ovarian follicles, in addition to an edematous corpus albicans and cystic change in the edematous stroma. Focal edematous change and occasional indefinite cystic structures, as seen in the left polycystic ovary of our patient, indicate an early stage of MOE. The pathogenesis of MOE in the present case was therefore suspected to be based on the underlying state of bilateral polycystic ovary. The luteinized cells would have been derived from granulosa cells and from the internal theca cells of the follicles in addition to stromal cells.