The clinical and histopathologic features of ten adenofibromas and 25 adenosarcomas of the uterus were studied. The most useful criterion for distinguishing adenofibroma from adenosarcoma was the frequency of mitotic figures found in the stroma. Adenofibromas had fewer than four mitotic figures per 100 HPF in the most active areas; adenosarcomas had four or more. Myometrial invasion, histologically malignant heterologous mesenchymal elements, and marked atypia of stromal cells were histologic features detected only in adenosarcoma. Of the women with adenosarcoma, ten (40%) had recurrences, with a median interval to recurrence of five years. The only morphologic feature that correlated with aggressive behavior of adenosarcoma was deep myometrial invasion. Adenofibroma and adenosarcoma are in the family of mixed mesodermal tumors, but are distinct clinical and pathologic entities.