One hundred cases of mullerian adenosarcoma of the uterus were encountered in patients 14 to 89 years of age (median, 58 years), who usually had the symptom of abnormal vaginal bleeding. An enlarged uterus and tissue protruding from the external os were the most common findings on pelvic examination. Five patients presented on multiple occasions with "recurrent polyps" that were interpreted retrospectively as adenosarcomas. Primary treatment, known in 97 cases, included some form of hysterectomy in 93 of them, and conservative resection in four cases. Gross examination of the excised uteri disclosed polypoid masses, some of which had spongy cut surfaces, usually filling the endometrial cavity; less commonly, the tumors were confined to the endocervix or the myometrium or involved more than one site. Histologic examination revealed benign or atypical neoplastic glands within a sarcomatous stroma, which typically formed periglandular cuffs of increased cellularity, intraglandular polypoid projections, or both. The sarcomatous stroma was homologous in 78% of the cases and contained heterologous elements in the remainder. The stromal mitotic rate varied from 1 to 40 mitotic figures (MFs) (mean 9) per 10 high-power fields (HPFs). Extensive areas of stromal fibrosis that focally imparted a deceptively benign appearance to the tumor were common. Myometrial invasion was present in 15 cases, but was deep in only four. Recurrent tumor developed in 23 cases at postoperative intervals of 0.5 to 9.5 years (mean 3.4); in one third of such cases, the interval was 5 years or longer. Recurrent tumor was almost always confined to the vagina, pelvis, or abdomen; hematogenous spread occurred in only two cases. The only feature associated with an increased risk for recurrence was the presence of myometrial invasion. Criteria found useful in separating mullerian adenosarcomas from mullerian adenofibromas included, alone or in combination: two or more stromal MFs per 10 HPFs, marked stromal cellularity, and significant stromal cell atypia.