BACKGROUND The transition of low-grade endometrial stromal sarcoma (ESS) to high-grade ESS remains a rare clinical event. METHODS A patient presented with abdominal pain and abnormal genital bleeding. She underwent a supracervical hysterectomy with bilateral salpingo-oophorectomy, omentectomy, and resection of peritoneal disseminated lesions. Pathological examination revealed low-grade ESS in the uterus and omentum. Immunohistochemical examination showed immunoreactivity for CD10 and Ki-67 (MIB1) in the uterus and omentum. However, estrogen receptor, progesterone receptor, alpha-SMA, desmin, h-caldesmon, and CAM5-2 were negative. P53 immunoreactivity was noted only in the omental lesion. Despite performing six courses of adjuvant chemotherapy, she recurred in the abdomen. She underwent ileostomy and resection of peritoneal disseminated lesions. Pathology showed high-grade ESS in the recurrent lesion of the ileum, which was characterized by severe cytologic atypia, high mitotic index, multifocal necrosis, increased Ki-67 index, and immunoreactivity for p53. CONCLUSIONS Although rare, the transition of low-grade ESS to high-grade ESS may occur and suggests the worsening of the prognosis. Pathological examination and immunohistochemistry are useful for the diagnosis of the transition of low-grade ESS to high-grade ESS.