The new histopathological classification of exocrine pancreatic tumors by the World Health Organization, now includes "ovarian-type stroma" ("OS") in the definition of mucinous cystic tumor of the pancreas (MCT-P). This study investigated the clinicopathological findings of the MCT-P according to WHO classification and scrutinize the function of "OS" in MCT-P immunohistochemically. Thirty-four cases of MCT-P (28 adenomas, 2 borderline tumors and 4 adenocarcinomas) were examined clinicopathologically. The "OS" of 34 MCTs-P were studied immunohistochemically and compared with the stroma of 10 mucinous cystic tumors of the ovary (MCTs-O), 10 conventional pancreatic carcinomas and 6 normal ovaries. Almost all 34 MCTs-P were located in the body-tail of the pancreas of middle-aged women. Histologically the "OS" cells exhibited luteinization in 11/34 (32.4%). Immunohistochemically, both "OS" cells and the stromal cells in MCT-O showed similar positivity rates; calponin (34/34 and 9/10), h-caldesmon (28/34 and 8/10), alpha-inhibin (29/34 and 9/10), estrogen-receptor (21/34 and 6/10) and progesterone-receptor (28/34 and 9/10, respectively). Some neoplastic epithelial cells of MCT-P were positive for human chorionic gonadotropin (hCG) (21/34, 61.8%). This study indicates the predominance of MCT in the distal pancreas of middle-aged women. Furthermore, the immunohistochemical and histological results demonstrate that the "OS" of MCT-P and the stroma of MCT-O share the same characteristics. The results also suggested that the hCG produced by the neoplastic epithelium probably plays an important role in the luteinization of the stroma of MCT-P. We therefore conclude there is a possibility that MCT-P originates from the left remnant primordial gonadal cells which migrated to the pancreatic anlage during the early development of the fetus.