We report a rare case of pancreatic carcinosarcoma in a 48-year-old man. An abdominal ultrasound scan incidentally detected a large heterogeneous mass in the left upper quadrant. CT and MRI scans indicated a rapidly growing unilocular cystic mass in the pancreatic tail. Grossly, the resected pancreatic mass measured 7×5×5 cm and consisted of a unilocular cystic lesion that had several solid mural nodules. Microscopically, the cystic wall showed two carcinomatous components, mucinous cystadenocarcinoma and anaplastic carcinoma. The solid lesion predominantly consisted of pleomorphic spindle cells arranged in interlacing fascicles which had infiltrated the adjacent pancreatic parenchyma. In some areas of this sarcomatous component, anaplastic carcinoma cells intermingled with or transformed into spindle cells. Immunohistochemically, the carcinomatous components were strongly positive for epithelial markers, and transforming anaplastic carcinoma cells were immunoreactive for both epithelial markers and vimentin; in contrast, the sarcomatous component was strongly positive for only vimentin. Additionally, an identical mutation (G to A transition) at codon 12 of K-ras gene and concordant immunoreactivity for p53 protein were detected in the carcinomatous and sarcomatous components. These findings suggested that the pancreatic carcinosarcoma could be of monoclonal origin, and that the sarcomatous component might have arisen from metaplastic transformation of the carcinomatous component.