A 26-year-old woman presented with pain in the left hypochondrium, for which pancreatectomy and splenectomy was performed, with total gross excision of a mass. A tumor measuring 11 × 9 cm was found in the pancreas. On cut surface there were two cysts filled with a necrotic substance and hemorrhagic content. Spindle or ovoid-shaped cells, in the sarcomatous component, had diffusely infiltrated along the inner side of the walls of one cyst. Osteoclast-like giant cells (OGC) were also contained in the sarcomatous component. Adenoma components of mucinous epithelium with foci of borderline and adenocarcinomatous components were seen on the inner side of the other cyst. An ovarian-type stroma beneath the epithelial component was seen in the cyst wall. A diagnosis of undifferentiated carcinoma with OGC arising in a mucinous cystic neoplasm (MCN) of the pancreas, was made. Seven months after the initial operation the patient had a local recurrence, and the tumor was removed. One month after the second operation, the patient was free of symptoms. Only four cases of undifferentiated carcinoma with OGC arising in MCN, involving an ovarian-type stroma of the pancreas, have been reported.