A series of 43 cases of uterine leiomyosarcoma was reviewed. Forty-one of the 43 patients were followed up 2 or more years. The overall 5-year survival was 73%. Favorable prognostic features included the premenopausal status of the patient, the confinement of the tumor within a myoma, the low mitotic count (less than four mitotic figures in any one high-power field or less than four mitotic figures per 10 high-power fields), the absence of necrosis, and the presence of hyalinization in the adjacent tissue suggesting confinement. Special cases such as those in which only myomectomy was performed and those receiving adjunctive chemotherapy are discussed. The differential in survival between patients with primary myosarcoma and those with lesions arising in a leiomyoma is stressed.