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From 2009 to 2016, 9 female patients with multicystic peritoneal mesothelioma (MCPM) underwent 11 cytoreductive surgeries (CRS). Mean age at diagnosis was 50.6 (range, 43-71). Mean peritoneal cancer index was 8.9 (range, 2~33). Most frequent peritoneal sector involved was pelvic peritoneum in 7 patients, and greater omentum was involved in 6 patients. Eight and 1 patients had complete CRS and incomplete CRS, respectively. Except 1 patient, 8 patients had multiple cysts on different peritoneal sectors, and diffuse involvement on peritoneal surface was found in 2 patients No patients had lymph node metastasis or extraperitoneal spread. However, 3 patients showed pushing invasion to small bowel mesentery, colon, umbilicus and stomach. Median- follow-up was 46 months (range 4-120). At the time of the present analysis, all patients were alive. One patient recurred in peritoneal cavity 47 month after complete cytoreduction. Peritoneal free-floating cysts (PFFC) were found in 8 of 9 (88.9%) patients. Sizes of PFFC ranged from 1 mm to 2 cm in diameter and the inner surfaces were covered with mesotheliallike cells. MIB-1 labeling rates of PFFC ranged from 0.1% to 2.0%. These results indicate that PFCC may attach on the orifice of subperitoneal lymphatic vessels, and may become metastatic. The present study strongly suggests that MCPM has a potentially malignant behavior. This category of disease is composed of 2 types: diffuse type and localized type with borderline malignant potential. PFFC have an important role in the formation of peritoneal metastasis.