Diffuse peritonitis due to perforation of pyometra is relatively rare. We experienced three cases of this type of peritonitis between 1993 and 1999. Three women aged 77, 83 and 91 years had mild abdominal pain, slight fever and defecation for nearly one week. Following these, severe abdominal symptoms developd. These patients underwent emergency laparotomy as they were diagnosed with diffuse peritonitis. At first, intestinal perforation was suspected, but perforation of pyometra was confirmed when lapasortomy was done. On preoperative examinations, physical findings were scanty in comparison with hemological ones, and free air on abdominal X-ray films was not detected. Thus, we failed to establish the preoperative diagnosis. The prognosis was very poor in two of the three patients. One was with advanced cervical cancer and the other was in multiple organ failure on admission. Differential diagnosis of this disease is important for the management of acute abdominal diseases in elderly women.