We investigated the value of prekeratin immunostaining in establishing the diagnosis of undifferentiated carcinoma of the nasopharyngeal type (lymphoepithelioma). As in squamous cell carcinoma of the nasopharynx, all seven lymphoepithelial carcinomas stained for prekeratin whereas other look-alike but histogenetically different neoplasms (malignant lymphoma, esthesioneuroblastoma, and small round cell sarcomas) did not stain. In addition to confirming the squamous epithelial nature of lymphoepithelioma, our findings indicated that immunoperoxidase staining for prekeratin is a useful diagnostic tool in the differential diagnosis of nasopharyngeal carcinomas in general and lymphoepithelioma in particular.