The incidence of leiomyosarcoma in uterine leiomyomas is estimated to be between 0.13 to 0.29%. However, the exact incidence of leiomyosarcoma in uteri removed with a preoperative diagnosis of benign uterine leiomyomas has not been previously reported. Between 1983 and 1988, a total of 1432 patients in Women's Hospital, a self-referred indigent population, had a hysterectomy planned because of abnormal uterine bleeding or abdominal pain associated with the presence of uterine leiomyomas, or because of a pelvic mass thought to be uterine leiomyoma of sufficient size or character to warrant surgical exploration. The ages of these women ranged from 36 to 62 years and the presence of leiomyosarcoma in the hysterectomy specimens increased steadily from the fourth to seventh decades of age (0.2%, 0.9%, 1.4%, and 1.7%, respectively). Preoperative histologic examination of the endometrium was performed in eight patients. Three of the eight patients had a preoperative tissue diagnosis of leiomyosarcoma that was clinically confined to the uterus. After the hysterectomy in the 1429 patients with presumed benign disease, histologic diagnosis of leiomyosarcoma was made in seven (0.49%). There was no evidence of malignancy in the endometrial sampling of any of these seven patients and the diagnosis was suspected intraoperatively in only three. Preoperative uterine size ranged from 8 to 20 weeks' gestational size and postoperative uterine weight ranged from 120 to 1100 gm. Seven of the 10 patients had symptoms of abnormal uterine bleeding. Between the ages of 40 and 60 years, 1% (8 of 817) of women with presumed uterine leiomyomas producing symptoms that necessitated hysterectomy in this series had leiomyosarcoma diagnosis postoperatively. Such treatments as gonadotropin-releasing hormone agonists, endometrial ablation, myomectomy by hysteroscopy or laparotomy instead of hysterectomy in such women could delay the diagnosis and definitive treatment of leiomyosarcoma.