Spontaneous perforation is a very rare complication of pyometra. We report herein the case of an 88-year-old woman who presented with muscular rigidity and free air on abdominal X-ray films. Perforation of the gastrointestinal tract was diagnosed preoperatively, and an emergency laparotomy was performed. A total hysterectomy with bilateral salpingo-oophorectomy was carried out under the diagnosis of generalized peritonitis caused by the spontaneous perforation of pyometra. The culture of purulent fluid from the abdominal cavity showed only Escherichia coli, with no anaerobic bacteria. Histological examination revealed pyometra with necrosis of the endometrium and no evidence of malignancy. The patient was discharged on postoperative day 68 without any major complications. Pyometra is an unusual cause of peritonitis, but it must be considered as a possible diagnosis in elderly women presenting with an acute abdomen. Following this case report, we discuss the problems associated with establishing a correct preoperative diagnosis of generalized peritonitis caused by the spontaneous perforation of pyometra.