Three cases of pulmonary blastoma (PB) were investigated microscopically with conventional stainings and immunohistochemically with monoclonal antibodies to cytokeratin, vimentin, desmin and neurofilament protein. The tumors differed in terms of morphology as well as of immunohistochemistry. Two were epithelial and mesenchymal mixed tumors, and the remaining one was a monophasic tumor of a typical blastemic character. The two mixed tumors also differed from each other. In one of them, the epithelial and mesenchymal component expressed cytokeratin and vimentin in a clear-cut manner without any transition. The other mixed tumor displayed a gradual epithelial-to-mesenchymal transition accompanied by a switch in the expression of cytokeratin and vimentin. The third tumor was of pure mesenchymal origin, expressing vimentin in the majority of cells and desmin in few cells. It is concluded that the PB is a morphologically and histogenetically heterogeneous tumor. Metaplastic changes may take place within a PB and make the recognition of embryogenesis more difficult.