Adenosarcoma is a biphasic neoplasm composed of benign epithelial and malignant stromal components. Adenosarcoma may involve the uterine corpus, cervix, vagina and, rarely, extrauterine sites. Clinically, most patients are postmenopausal and present with vaginal bleeding, an enlarged uterus, a pelvic mass, or a polypoid lesion protruding through the cervical os. Microscopically, variably dilated (rigid cysts) glands are lined by benign Mullerian (endometrial-type, endocervical-type mucinous or tubal-type ciliated) epithelium and surrounded by hypercellular stromal cells (periglandular cuffing). Intraglandular projections of broad, leaf-like structures, resembling phyllodes tumor of breast are often seen. In low-grade adenosarcomas, the stromal cells show low-grade atypia with minimal to absent mitotic activity. However, the sarcomatous component may exceed 25% of overall tumor volume (so-called stromal or sarcomatous overgrowth) or may undergo transformation to high-grade sarcoma. The sarcomatous component is typically homologous type but may show heterologous (such as rhabdomyosarcomatous) differentiation. The epithelium often exhibits metaplastic changes. Rarely, atypical hyperplasia or low-grade endometrioid adenocarcinoma may arise in adenosarcoma. Immunohistochemistry is not usually required for diagnosis. The stromal cells are variably positive for CD10, ER, and PR. Rhabdomyosarcomatous differentiation may be confirmed by myogenin or Myo-D1. The prognosis is variable. Predictors of poor prognosis include sarcomatous overgrowth, deep myometrial invasion, high-grade atypia, and extrauterine recurrence. Unlike pure uterine sarcomas, adenosarcomas are staged based on the depth of myometrial invasion, not tumor size.