PURPOSE: To evaluate the imaging findings of endometrial stromal sarcoma (ESS) according to histopathologic grade. MATERIALS AND METHODS: Six patients with pathologically proven ESS were included in this study. The histopathologic diagnosis was low-grade ESS for three patients and high-grade ESS for the three others. Preoperative CT or MR images were evaluated in terms of tumor size, location, growth pattern, the presence of hemorrhage or necrosis, status of the endometrial cavity, and invasion of surrounding structures. The imaging features of ESSs, which varied according to their histopathologic grade, were compared. RESULTS: The mean maximal diameter of low-and high-grade ESSs was 6 cm and 11.2 cm, respectively. All three low-grade ESSs were located mainly in the myometrium, but two high-grade ESSs were situated in the endometrial cavity and associated with focal tumor extension into the myometrium. One high-grade ESS had completely replaced the uterus. Low-grade ESSs were relatively well-defined, but high-grade ESSs had an irregular and lobulated margin. Intratumoral hemorrhage and necrosis were, respectively, found in two and three high-grade ESSs. Widening of the endometrial cavity was noted in all three high-grade ESSs, and lymph node metastasis had occurred in one. CONCLUSION: The imaging findings of ESS vary from a well-defined intramural mass to a bulky infiltrating mass, and depend on their histopathologic grade.