Mucinous cystic neoplasm(MCN)of the pancreas is a rare disease. A 34-year-old female was referred to our hospital for a giant cystic tumor in the left epigastrium, suspected of being a pancreatic MCN. The surgical findings revealed that the tumor originated in the pancreatic tail with the presence of peritoneal dissemination. A distal pancreatectomy and a splenectomy were performed, and the resected specimen histologically revealed an invasive mucinous cystadenocarcinoma of the pancreas. The postoperative computed tomography(CT)scan showed metastatic tumors of the Douglas pouch and the left ovary. Gemcitabine(GEM)was thereafter systemically administered for palliative chemotherapy with a regimen of 1,000 mg/m / 2week for 3 weeks, followed by a week of rest. When assessed by a CT scan after 4 courses of chemotherapy, marked shrinkage of the tumors was identified, and we could not detect the tumors clearly. Moreover, the serum CA19-9 level fell from 341 U/mL to almost normal and there were no severe adverse events. Therefore, systemic chemotherapy with GEM is considered to possibly be an effective treatment against MCN. We describe herein the first case of advanced mucinous cystadenocarcinoma of the pancreas with peritoneal dissemination responding to GEM and a brief review of the literature.