To address the premise that pulmonary "carcinosarcomas" and spindle-cell carcinomas are part of a single clinicopathologic continuum, the authors studied 21 examples of such lesions as defined by World Health Organization criteria. Two biphasic tumors demonstrated an admixture of overt carcinoma with other foci showing partial rhabdomyogenic differentiation; 15 others were histologically similar but lacked "heterologous" sarcoma-like elements; and four lesions were monophasic spindle-cell sarcomatoid carcinomas. One of the latter also contained rhabdomyosarcoma-like areas by light microscopy. Sarcomatoid components were reactive for keratin and/or epithelial membrane antigen (EMA) in 18/21 cases. In addition, desmin and muscle-specific actin were co-detected in the same spindle cells that were keratin-positive in 4 tumors, 3 of which were those with partially myogenic histologic features. Vimentin was present in keratin- or EMA-reactive sarcomatoid cells in 12 neoplasms, and all cases were labeled with an antibody to collagen type IV. Survival was poor in this group of patients; only 1 was alive at last contact. These data support the contention that "carcinosarcoma" of the lung is part of a spectrum with "spindle-cell carcinoma." It is proposed that the terms "biphasic sarcomatoid carcinoma" and "monophasic sarcomatoid carcinoma" are more apt descriptors for such tumors.