BackgroundPyosalpinx, which is one of the pelvic inflammatory diseases, is usually observed in young women; it is rarely found in older women. Possible causative agents are thought to be Chlamydia trachomatis and Neisseria gonorrhea in addition to some Enterobacteriaceae. On the other hand, type 2 diabetes is a disease with a lot of complications such as hyperglycemia, inflammation, and immune disorders. Therefore, patients with type 2 diabetes mellitus have an increased susceptibility to infection especially when glycemic control is poor.Case presentationWe experienced a rare case of large pyosalpinx in an elderly patient with well-controlled type 2 diabetes mellitus. A 72-year-old Japanese woman with a 10-year history of type 2 diabetes mellitus had symptoms of diarrhea and persistent pain in left lower abdomen. She had mild tenderness to palpation in her abdomen. Inflammation markers were markedly elevated. Her abdominal computed tomography and magnetic resonance imaging on admission revealed a tumor in left side of intrapelvis and we diagnosed her as having pyosalpinx. Pathogenic bacteria were not detected. On admission, her glycemic control was relatively good; in addition, her glycated hemoglobin levels were around 6% for over 1 year. Although pathogenic bacteria were not detected, we started antibiotics therapy. Fourteen days after starting the antibiotics her laboratory data were improved. Three months later, the tumor was markedly smaller compared to that on admission.ConclusionsWe should keep in mind that older patients with type 2 diabetes mellitus are immunocompromised hosts and thereby they could have rare pelvic inflammatory disease such as pyosalpinx even when good glycemic control is obtained for a long period of time.