Benign multicystic peritoneal mesothelioma(BMPM)is a rare cystic mesothelial lesion that occurs predominantly in reproductive aged women.A 56-year-old Caucasian male was admitted to our surgical department with a chief complaint of a painful mass in his right lower abdomen for almost 2 years.The physical examination revealed a palpable painful mass.Computed tomography demonstrated an irregular,cystic tumor in his right lower abdomen.There was no obvious capsule or internal septations.No enhancement after intravenous administration of contrast was noted.An exploratory laparotomy was performed,and a multicystic tumor and adherent to the caecum was noted.The walls of the cysts were thin and smooth,filled with clear fluid,and very friable.An en bloc resection of the tumor,including appendix and caecum,was performed.Histological examination revealed multiple cysts lined with flattened simple epithelial cells,and the capsule walls of the cysts were composed of fibrous tissue.Immunohistochemical analysis documented positive expression of mesothelial cells and calretinin.The final diagnosis was BMPM.The patient was well at 6-mo follow-up.BMPM is exceedingly rare lesion.A complete resection of the tumor is required.The diagnosis of BMPM is based on pathological analysis. Benign multicystic peritoneal mesothelioma (BMPM) is a rare cystic lesion that occurs predominantly in reproductive aged women. A 56-year-old Caucasian male was admitted to our surgical department with a chief complaint of a painful mass in his right lower abdomen for almost 2 years.The physical examination revealed a palpable painful mass. Computed tomography demonstrated an irregularity, cystic tumor in his right lower abdomen. There was no obvious capsule or internal septations. No enhancement after intravenous administration of contrast was noted. An exploratory laparotomy was performed , and a multicystic tumor and adherent to the caecum was noted. walls of the cysts were thin and smooth, filled with clear fluid, and very friable. Ann bloc resection of the tumor, including appendix and caecum, was performed. Histological examination revealed multiple cysts lined with flattened simple epithelial cells, and the capsule walls of the cysts were composed of fibrous tissue. Immunohistochemi cal analysis documented positive expression of mesothelial cells and calretinin. The final diagnosis was BMPM. The patient was well at 6-mo follow-up. BMP was isly lyre lesion. A complete resection of the tumor is required. The diagnosis of BMPM is based on pathological analysis.