A 25-yr-old woman delivered a healthy child by cesarean section. At 8 mo postpartum, she became aware of an upper abdominal tumor. Abdominal computed tomography and upper abdominal ultrasonography revealed a large cystic mass in the body of the pancreas. Endoscopic retrograde pancreatography showed no connection between the main pancreatic duct and the cystic lesion. The patient underwent tumor resection at 11 mo postpartum. Pathological examination of the tumor revealed mucin-producing columnar epithelial cells lining the cystic wall with ovarian-type stromal tissue and no findings indicative of malignancy, giving a diagnosis of mucinous cystic adenoma of the pancreas. Immunohistochemical studies revealed positive staining for progesterone receptor but not for estrogen receptor in the stromal cell nuclei. Postpartum rapid growth of a benign mucinous cystic neoplasm might be linked to the production of female sex hormones during lactation.