Undifferentiated carcinoma of the nasopharynx has a well-known association with Epstein-Barr virus (EBV), but only an inconsistent relationship has been identified in undifferentiated carcinomas occurring at other sites. We investigated 22 formalin-fixed, paraffin-embedded cases of sinonasal undifferentiated carcinomas (SNUCs) occurring in Western and Asian patients. A highly sensitive in situ hybridization method was performed using an antisense oligonucleotide probe to the EBER1 gene of EBV. We identified EBV RNA in seven of 11 SNUCs from Asian patients, but in none of the Western SNUC patients (0/11). The EBER1 signal was present in all or virtually all of the tumor cell nuclei in the seven EBV-RNA-positive Asian SNUCs. The latent membrane protein-1 (LMP1) of EBV was not identified in any of the five positive cases tested. Our results suggest that genetic predisposition or environmental/geographical cofactors play an important role in determining the strength of the association of SNUC with EBV.