Mullerian adenosarcomas are rare mixed tumors of low malignant potential that occur mainly in the uterus and also in extrauterine locations. Microscopically, they may be difficult to distinguish from adenofibromas. In this clinicopathologic study of 55 adenosarcomas, the mean patient age was 50 years (range: 13 to 83 y). Thirty-seven tumors were of the uterine corpus, 11 of the cervix, 4 of the ovary, and 1 each of the fallopian tube, vagina, and Douglas peritoneum. Abdominal pain and vaginal bleeding were the usual complaints. Treatment was known in 50 patients: 10 had polypectomy, 1 cone biopsy, and 39 hysterectomy, which was accompanied by bilateral salpingo-oophorectomy in 24 and lymphadenectomy in 4. Five patients had radiotherapy and 2 of them had chemotherapy. Stage was known in 41 cases. Of 30 tumors of the uterine corpus, 17 were stage IA, 11 stage IB, 1 stage IC, and 1 stage IIIC. Four cervical tumors were stage IB. Three of the 4 ovarian tumors were stage IA and the other was stage IIIC. The tumor of the fallopian tube was stage IC, and the tumors of the vagina and recto-uterine pouch were confined to their site of origin. Most uterine tumors were polypoid masses ranging from 1 to 20 cm (mean: 6.5 cm). Microscopically, sarcomatous overgrowth was found in 18 cases (33%), heterologous elements in 13 (24%), and sex cordlike differentiation in 7 (13%). Fourteen of 30 uterine tumors (47%) had myometrial invasion that was minimal in 5, involved one-third of the myometrial thickness in 7, and more than 50% in 2. Of 4 cervical tumors, 2 were endocervical polyps, 1 invaded one-third of the cervical wall, and the other invaded its full thickness. Follow-up information (2 mo to 18 y; average: 7.5 y) was available in 29 patients. Six developed metastases and 5 of them died of tumor. Four had adenosarcomas with sarcomatous overgrowth; however, the other 2 patients had typical low-grade adenosarcomas of the uterine corpus and cervix, respectively, exhibiting only mild nuclear atypia of the stromal component and </=2 mitotic figures/10 high power fields. Both were initially underdiagnosed as adenofibromas. The finding of such cases, which raises the controversy of whether or not adenofibroma exists as a tumor entity, prompted us to make a comparative immunohistochemical analysis of 23 typical adenosarcomas, 8 adenosarcomas with sarcomatous overgrowth, and 29 benign and malignant related lesions, including 7 clinically benign adenofibromas. Adenosarcomas with sarcomatous overgrowth showed strong immunoreaction for Ki-67 and p53 and loss of CD10 and progesterone receptors immunostaining; in contrast, the immunoreaction for these tumor markers in typical adenosarcomas without sarcomatous overgrowth was similar to that of adenofibromas associated with favorable outcome and other benign lesions such as endometrial polyps and endometriosis. These findings suggest that some of the tumors currently classified as adenofibromas, on the basis of their low mitotic count and lack of significant nuclear atypia, are, in fact, well-differentiated adenosarcomas.