Six patients treated with leuprolide acetate for rapid uterine growth caused by presumptive uterine fibroids were found to have malignancy during subsequent laparotomy. Two patients had endometrial stromal sarcoma; three had leiomyosarcoma; and one had a cervical lymphoma. Five of the six patients had significant reduction in fibroid size before laparotomy and most had improvement in bleeding. Five of the six had an endometrial sampling done before treatment and most had an office-based sonography only. Three patients had an intraoperative frozen section. Appropriate pretreatment evaluation of patients with rapid uterine growth before leuprolide acetate therapy must include an endometrial biopsy and accurate uterine sonography.