Carcinosarcoma and spindle cell carcinoma of the lung. Clinicopathologic and immunohistochemical studies. We examined pulmonary carcinomas with prominent sarcoma-like lesions both clinicopathologically and immunohistochemically. Grossly, two tumors had predominantly endobronchial growths, four bulky parenchymal growths, and two endobronchial, parenchymally mixed growths. In these eight patients, six tumors were completely resected, one patient was given irradiation only, and one patient died in the early postoperative period. On the basis of specific differentiation of the sarcoma-like lesions, the tumors were separated into three groups: two with "true" sarcoma differentiated into soft tissues such as striated muscle or osteoid tissue; three with a fibromatous sarcoma resembling atypical pseudosarcomatous stroma; and three with spindle cell carcinoma with evidence of epithelial differentiation. The prognosis was poor, and tumors with specific differentiation into rhabdomyosarcoma, chondrosarcoma, or spindle cell carcinoma progressed more rapidly than did those with a fibromatous sarcoma. Because the fibromatous sarcoma-like lesions were found to relate to a longer survival time for the patients, we wish to emphasize that a distinction of sarcomatous components should be made with regard to assessing the prognosis of pulmonary carcinoma with sarcoma-like lesions. 