In order to clarify the histogenesis and the direction of differentiation of spindle-cell and sarcomatous components of esophageal carcinosarcoma, 20 cases of the disease were reviewed histologically and immunohistochemically using the avidin-biotin-peroxidase complex method with monoclonal and polyclonal antibodies to various keratins, vimentin, desmin, muscle specific actin and S-100 protein. A gradual transition between carcinomatous and spindle cell sarcomatous components was present histologically in all 20 cases. Positive immunoreactivity for keratins was found in carcinomatous areas in all cases. Spindle cells in the transitional areas were positive for keratins in nine cases and for vimentin in five. Two cases demonstrated trace positive reactions to both keratin and vimentin in the same areas of transitional spindle cells between carcinomatous and sarcomatous components. The sarcomatous component showed an immunohistochemically positive reaction for vimentin in ten cases and for desmin in two. In one of the 20 cases, chondrosarcomatous cells were seen which showed a positive reaction to S-100 protein but were negative to keratin. The findings strongly suggested that neoplastic epithelial cells may show dedifferentiation to transforming spindle cells and also disdifferentiation to non-epithelial sarcoma like chondrosarcoma and leiomyosarcoma.