Spindle cell carcinoma of the lung. A clinicopathologic study of three cases. Two cases of monophasic spindle cell carcinomas and one case of adenosquamous carcinoma with the spindle cell component located in the lower respiratory tract are presented. In the biphasic tumor, areas of transition from carcinoma to sarcomatous spindle cells were clearly found. The two monophasic tumors and the spindle cell component of the biphasic tumor were histologically characterized by sheets of spindle cells. However, by electron microscopic and immunohistochemical study, several features of squamous epithelial differentiation were found in the spindle cell areas of all cases. Keratin and vimentin were, in various degrees, coexpressed in all the cases. Therefore, it is supposed that the spindle cell component displays a spectrum of phenotypes originating from squamous cell carcinoma, and monophasic spindle cell carcinoma is considered as a kind of the extreme phenotype of squamous cell carcinoma pretending mesenchymal differentiation. 