OBJECTIVE: The behavior of endometrial stromal sarcoma (ESS) is still not clear. We report our results about clinical and pathologic features of endometrial stromal sarcoma to understand their clinical behavior. MATERIALS AND METHODS: 11 patients who were diagnosed as ESS at Asan Medical Center from Jun., 1989 to Oct., 2001 were included in this study. The FIGO classification for endometrial stromal sarcoma was used in this study. All the clinical data were obtained retrospectively from medical records. RESULTS: The median follow-up time was 21.3 months (range; 2 to 61 months). The median duration of disease free survival was 16.7 months (ranges : 2-61 months). The mean age of the patients was 46.7 years (range: 23-63). The most common preoperative diagnosis was myoma of the uterus.(7/11 patients). The others were endometrial polyp, cervical cancer, and ESS. Only one patient was proved to be ESS by cervical punch biopsy which was taken prior to the operation. Eight patients were in stage I and two in stage III. One could not be evaluated by stage, as she had taken operation at the other hospital without clear record of it. Ten patients were low grade ESS. Six patients were treated only by surgery in a curative intent. Five patients received postoperative chemotherapy, and no patient received radiotherapy. Eight out of eleven patients were alive without any evidence of disease and two were in the state of stable disease. One patient was dead from ESS. Three patients experienced recurrences after adjuvant chemotherapy. The two patients (2/10 low grade ESS) with low grade ESS were recurred and one in high grade ESS(1/1 high grade ESS). CONCLUSION: It is very difficult to diagnose ESS preoperatively. Clinical and pathological features have broad spectrum of characteristics. Surgery and adjuvant chemotherapy is the most effective treatment for patients with ESS. Less myometrial invasion, earlier tumor stage, less mitotic count are associated with longer overall survival in patients with ESS.