The case histories of 15 patients with endometrial stromal sarcoma attending the Royal Marsden Hospital between 1973 and 1987 were reviewed. Eight patients were classified as having low-grade and seven as high-grade tumors using specific histological criteria. Those patients with low-grade tumors had a long relapse-free and overall survival in the absence of receiving specific therapy after initial surgery. Five are alive and disease free, one has shown a partial response to hormonal manipulation, and one is alive with disease on no therapy. Only one patient has died and this occurred 111 months postdiagnosis. Conversely, all seven patients with high-grade tumors relapsed with local disease and three have died. Thus, after surgical resection patients with low-grade tumors probably do not require treatment until relapse, whereas adjuvant radiotherapy should be given to those patients with high-grade tumors. On relapse, hormone therapy should be the treatment of first choice for patients with low-grade tumors, whereas chemotherapy is more appropriate for patients with high-grade tumors.