The Epstein-Barr virus (EBV) has been detected in certain types of lymphoma and some epithelial neoplasms such as nasopharyngeal lymphoepithelioma and occasional undifferentiated carcinomas in several organs including the salivary glands. However, clonal EBV genomes have been detected in undifferentiated carcinomas of the parotid gland exclusively in Alaskan natives and Eskimos, both groups being at the highest risk for nasopharyngeal carcinoma. The authors investigated the possibility that EBV may be present in undifferentiated parotid carcinomas in Caucasian subjects. To test this hypothesis, in situ hybridization (ISH) technique with biotinylated EBV-DNA probes was utilized on routinely processed, paraffin-embedded tissues from 7 cases of undifferentiated carcinomas of the parotid gland. EBV genomes were demonstrated in the cytoplasm of tumor cells from 3 out of 7 specimens tested. Surprisingly, EBV genomes were found in 3 out of 5 (60%) undifferentiated carcinomas that had developed in patients with a history of a long-persisting asymptomatic parotid mass, which had suddenly increased in size. Conversely, none of the undifferentiated carcinomas with continuous and rapid growth studied was found to be positive for EBV-DNA by ISH technique. Taken together, these data might suggest a possible role of EBV in the transformation of benign parotid gland lesions into malignant and aggressive undifferentiated carcinoma of the parotid gland, the so-called carcinoma expleomorphic adenoma.