Background: Spontaneous perforated pyometra presenting as pneumoperitoneum is extremely rare. Case report: An 80-year-old Japanese female with spontaneous perforating pyometra presenting as pneumoperitoneum is reported. The patient came to our institute with severe abdominal pain. Routine abdominal examination showed muscular defense, and plain chest roentgeno-grams revealed infradiaphragmatic free gas. Subsequent computed tomography also demonstrated pneumoperitoneum. Laparotomy was performed on the basis of a tentative diagnosis of perforation of the gastrointestinal tract but revealed a perforated pyometra. A simple hysterectomy was performed. The histological diagnosis of the surgical specimen was acute endometritis without neoplasm. The present report is the third case of spontaneous perforated pyometra with pneumoperitoneum to date. Conclusion: Although uterine disease presenting as pneumoperitoneum is rare in elderly patients with an acute abdomen, the possibility of a perforated pyometra should be considered in the differential diagnosis.