Mullerian adenosarcoma is an uncommon, but not rare, mixed tumor containing a neoplastic but benign or mildly atypical epithelial element and a sarcomatous, usually low-grade, stromal component. The most common site is the uterine corpus but adenosarcoma also occurs in the cervix and ovary and more rarely in the vagina, fallopian tube, arising from peritoneal surfaces, or outside the female genital tract, for example in the intestine. Most uterine cases have a polypoid gross appearance, sometimes resulting in the formation of multiple polyps. Characteristic histologic features include a low power "phyllodes-like" architecture with leaf-like projections lined by a variety of benign Mullerian type epithelia, sometimes with squamous metaplasia. Intraglandular stromal protrusions are a characteristic feature. The stroma may be uniformly cellular but there is typically increased cellularity around the epithelial elements, resulting in the formation of a cambium layer. Using the World Health Organization definition, stromal mitotic activity of 2 or more per 10 high-power fields is required for a diagnosis of adenosarcoma but in practice the diagnosis is made with stromal mitotic activity less than this if the characteristic architecture and cambium layer is present. The stromal component is usually morphologically "low-grade" and of endometrial stromal or fibroblastic type (hormone receptor and CD10 positive). Sometimes it is high grade, resembling undifferentiated sarcoma. Additional features sometimes present include heterologous stromal elements or sex cord-like differentiation. Uterine adenosarcomas are, in general, low-grade neoplasms capable of local recurrence after polypectomy or hysterectomy and much less commonly distant metastasis. The 2 most important adverse prognostic factors, which sometimes coexist, are deep myometrial invasion and sarcomatous overgrowth; the latter is usually associated with morphologically "high-grade" stromal elements with loss of expression of hormone receptors and CD10. Adenosarcoma may be confused with a variety of lesions and one of the main differential diagnoses is adenofibroma in which the stromal component is, by definition, morphologically benign. However, occasional adenofibromas recur or even metastasize. As such, it has been suggested that all adenofibromas should be classified as adenosarcomas, albeit with low-malignant potential. Ovarian adenosarcomas are much more likely to exhibit malignant behavior than their uterine counterparts, probably due to the lack of an anatomic barrier to peritoneal dissemination.