This is a light- and electron-microscopic study of a large ovarian adenosarcoma in a 50-year-old woman as well as a review of the literature. The tumor showed a variety of müllerian type epithelia and a spectrum of stromal patterns including nonspecific fibroblastic, and endometrial type stroma. The latter ranged from normal-appearing to low-grade sarcoma. This study emphasizes the relationship of ovarian adenosarcoma to cystadenofibroma, on the one hand, and to malignant mixed müllerian (mesodermal) tumor, on the other. It appears that the mitotic count which is used as the primary means for the classification of smooth muscle and stromal tumors of the uterus, has less value in the assessment and prognostication of ovarian adenosarcomas.