The purpose of this study was to evaluate the activity of ifosfamide (isophosphamide) in patients with advanced or recurrent leiomyosarcoma not previously exposed to chemotherapy. This is a phase II groupwide study of the Gynecologic Oncology Group. Thirty-five patients were treated with ifosfamide 1.5 gm/m2 daily intravenously for 5 days with mesna (mercaptoethane sodium sulfonate). Fifty-six patients had received prior abdominal hysterectomy and 15 prior radiotherapy. The dose was reduced to 1.2 gm/m2 daily in patients who had received prior radiotherapy. Gynecologic Oncology Group grade III or IV granulocytopenia occurred in 4 (11%) patients, and none had grade IV thrombocytopenia. One (2.8%) patient had grade IV neurotoxicity. Partial responses were observed in 6 of 35 (17.2%) patients. The 95% confidence interval for response was 6.6% to 33.7%. These results indicate that ifosfamide has modest activity in patients with advanced or recurrent leiomyosarcomas of the uterus.