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Well-differentiated papillary mesothelioma (WDPM) is an uncommon mesothelial tumor that occurs in the peritoneum of women over a wide age range. Although considered a tumor of uncertain malignant potential, information about its biological behavior is still limited. In this study, we present the clinicopathologic features of 26 cases of WDPM of the female peritoneum seen in our institution over a 20-year period (1990 to 2010). Clinical information and pathology material were reviewed in all cases. Patients ranged in age from 23 to 75 years (median, 47 y; mean, 48.6 y). There was no history of asbestos exposure in any of our cases. Ten patients had undergone surgery previously, and 6 had a history of endometriosis. In 24 patients, the WDPM was an incidental finding during surgery for a benign or malignant lesion. Only 2 patients presented with symptoms: 1 with an acute abdomen and the other with chronic pelvic pain. The former had developed a small hemoperitoneum because of bleeding of 1 of the lesions of WDPM, whereas the latter had a 2-cm WDPM involving the distal fallopian tube. The lesions were single or multiple (13 cases each) and ranged in size from 0.1 cm to 2 cm. The following sites were involved: abdominal or pelvic peritoneum not otherwise specified (10 cases), omentum (7 cases), cul-de-sac (6 cases), colonic serosa (4 cases), small bowel mesentery (2 cases), uterine serosa (2 cases), stomach serosa (1 case), large bowel mesentery (1 case), fallopian tube (1 case), ovary (1 case), and inguinal hernia (1 case). In all cases the lesions were excised. Microscopically, all of our cases had the typical features described for WDPM (ie, a papillary architecture that may be accompanied by glandular/tubular patterns, nests of cells and individual cells, bland mesothelial cells, absent or rare mitotic figures). The initial diagnosis in our cases was variable, including WDPM, mesothelial hyperplasia, malignant mesothelioma, serous tumor of low malignant potential of the peritoneum, papillary endosalpingiosis, and chronic xanthogranulomatous salpingiosis. Follow-up was obtained for 25 patients, and it ranged from 4 to 192 months (mean, 47.5 mo; median, 32 mo); 22 patients are alive with no evidence of WDPM after a follow-up that ranged from 5 to 144 months. One of these patients experienced recurrence of WDPM 46.5 months after initial diagnosis. In this patient, WDPM was an incidental finding during a total abdominal hysterectomy and bilateral salpingo-oophorectomy for serous cystadenofibroma. The recurrence was also an incidental finding during a colectomy for colonic adenocarcinoma. This patient is alive with no other recurrences 73 months after initial diagnosis and 36 months after diagnosis of the recurrence. Three patients died of other causes: pancreatic cancer at 4 months and 12 months and leukemia at 192 months. Recognition of the histologic features of WDPM and proper clinical correlation allow for the correct diagnosis of this entity. If necessary, immunohistochemical studies such as calretinin and keratin 5/6 facilitate the recognition of the mesothelial nature of this neoplasm. Although no patient died of disease in this series, follow-up of patients with this diagnosis is warranted on the basis of possible recurrences or misdiagnosis of an undersampled malignant mesothelioma.