A laryngeal squamous cell carcinoma in situ with an underlying spindle cell nodule (pseudosarcomn) was immunohistochemically labeled with antibodies to tissue-specific intermediate filament proteins, including desmin, vimentin, and cytokeratin. Two distinct populations of cells were found within the lesion: cytokeratin-positive cells, corresponding to the carcinomatous component of the tumor, and vimentin-positive spindle cells in the subepithelial nodule. In view of the strict specificity of antivimentin and an- ticytokeratin for cells of mesenchymal and epithelial origin, respectively, it is proposed that the two components of the pseudosarcoma in our case are not morphologic variants of the same tumor, and that the subepithelial nodule represents a mesenchymal lesion. These results can, however, not be extrapolated to other cases since in some the spindle cell component may represent metaplastic epithelial cells. In view of the difficulties encountered in reaching a correct diagnosis in these lesions, it is recommended to use intermediate filament typing to elucidate the nature of the spindle cells in this controversial tumor.