Pulmonary carcinosarcoma, consisting of both carcinoma and sarcoma with a heterologous element, is a rare subtype, comprising approximately 0.3% of primary lung neoplasia. A 57-year-old man was admitted because of severe dyspnea. A tumor wholly occupying the right thorax was biopsied and diagnosed as pleomorphic sarcoma. The tumor did not respond to chemotherapy, and the patient died of respiratory failure and sepsis. At autopsy, pleomorphic sarcoma was histologically dominant and contained a liposarcoma element confirmed by histocytological and electron microscopic analysis. Adenocarcinoma component with papillary and tubular patterns was confined to the medial lesion of the right lower lobe (3 × 8 cm), which was found in the chest X-ray 3 years before admission, and had continuously merged with the sarcomatous lesion through the histological transition of both components. Aggressive and rapid growth of the sarcoma derived from the earlier adenocarcinoma became prevalent and contributed to the severe clinical outcome. This is the first documented case of primary lung carcinosarcoma with a liposarcoma element.