We report three cases of adenosarcomas arising from extraendometrium of the uterus: one arising from the ovary, one from the paracolpium and one from the endocervix of the uterus. Microscopically, they consisted of an admixture of benign-appearing epithelial and mesenchymal components with hypercellularity and minimal atypia. Two of the tumors were initially misdiagnosed as endometriosis and one was diagnosed as adenofibroma. One patient had several recurrences and died 7 years after the initial laparotomy and another patient had sarcomatous overgrowth which invaded the muscular tissues of the large intestine. Thus it appears that adenosarcoma occasionally shows grave clinical behavior, despite the benign or low-grade appearance of its microscopic features. Problems of diagnosis and management of this tumor are discussed. An aggressive therapeutic approach including wide surgical excision is recommended even in questionable cases.