To elucidate the possible role of Epstein-Barr virus (EBV) in nasopharyngeal carcinoma (NPC), we investigated the EBV genome in NPC by in situ DNA hybridization using radioisotope- and and biotin-labeled EBV DNA probes. The EBV genome was detected in the tumor cells in all (100%) 60 cases, irrespective of histological type, but not in the lymphocytes. Silver grains, which reflected the copy number of the EBV genome, were more abundant in the nonkeratinizing, spindle cell, and undifferentiated carcinomas than in keratinizing squamous cell carcinoma. In the keratinizing carcinoma, which was poorly differentiated in this series, the EBV genome was usually detected in anaplastic tumor cells, and not in the keratinizing areas. The sensitivity of the radioisotopic technique was superior to that of the biotinylated probe method (100 vs. 81.7%, p < 0.0003). These results suggest that EBV is etiologically related to nasopharyngeal carcinogenesis and that the differentiation of tumor cells in vivo, and probably also in vitro, may become incompatible with EBV replication.