Thirty-seven cases of uterine leiomyosarcoma are presented, along with nine cases of leiomyoma variants from which they were distinguished. All patients were followed for a minimum of 10 years. In cases with nuclear pleomorphism, leiomyosarcoma was diagnosed when there were five or more mitotic figures in ten consecutive high-power (X400) fields in the most active area of the tumor, and also when there were fewer mitotic figures but extensive tumor necrosis (there was only one leiomyosarcoma without nuclear pleomorphism, and it had more than 20 mitotic figures in ten high-power fields). Tumor size was the major prognostic factor in the leiomyosarcoma group; five of eight patients with neoplasms measuring less than 5 cm in maximum dimension survived, whereas no patient with a larger tumor did so. Other pathologic and clinical variables, including mitotic rate, tumor necrosis, degree of nuclear pleomorphism, vascular invasion, and patient age, had no significant relationship to survival or tumor behavior in leiomyosarcoma when tumor size was taken into account. The nine cases of leiomyoma variants consisted of three atypical leiomyomas (which had nuclear pleomorphism, one or no mitotic figures per ten high-power fields, and no necrosis), two plexiform leiomyomas, two cases of peritoneal leiomyomatosis, one palisaded leiomyoma, and one case of intravenous leiomyomatosis; all of these patients were tumor-free on follow-up.