METHODS: We report a rare and difficult case of peritonitis in an elderly female that was caused by a spontaneous perforation of pyometra. A 90-year-old postmenopausal woman was referred to our hospital with complaints of vomiting, fever, and abdominal pain. Computed tomography revealed a large amount of ascites, cystic mass in the uterus, and intraperitoneal and intrauterine air. Transvaginal ultrasound demonstrated a thin area around the fundus. An emergency laparotomy was performed for the suspected gastrointestinal perforation or perforation of pyometra. At laparotomy, copious purulent fluid was present in the peritoneal cavity; however, no perforation of the gastrointestinal tract was observed. We identified a perforation site over the uterine fundus and purulent material exuding from the cavity. Subsequently, hysterectomy and bilateral salpingo-oophorectomy were performed. The patient was discharged on postoperative day 13 with no complications. Histopathologic studies revealed endometritis and myometritis with no evidence of malignancy. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: With diffuse peritonitis, ruptured pyometra should be considered, even in elderly female patients. This case illustrates the importance of clinical knowledge of acute gynecologic diseases. Here we also review the perforation of pyometra with no evidence of malignancy.