Seven cases of nasal lymphoma were studied to identify the lineage of Epstein-Barr virus (EBV)+ cells using dual-labelling methods. Five cases were phenotypically and genotypically of natural killer cell (NK) type with germ-line configuration of T-cell receptor (TcR) β-chain gene and immunoglobulin heavy-chain joining region (lg/H) gene, with one case each of T- and B-cell type showing rearranged TcRβ or lg/H and Λ-light chain genes respectively. EBV genome was clonal in all these cases except in the B-cell case where its clonality was undeterminable. Using in situ hybridization (ISH) for EBV-encoded small nuclear RNA 1 and 2 (EBER), signal was detected in 45% to 88% of nucleated cells in the tumours. Immunostaining for EBV latent membrane protein-1 (LMP) also revealed numerous LMP+ cells in 3/5 NK-type cases and the T- and B-cell cases. Using ISH for EBER combined with immunostaining for CD markers and double immunohistochemistry for LMP and CD markers, the predominant lineage of the EBV+ cells was identified as: CD2+CD3−CD19−CD20- CD45R0+CD56+CD68- in the NK-type cases, CD2+CD3+CD19−CD20− CD45R0+CD56−CD68− in the T-cell case and CD20+CD45R0−CD68- in the B-cell case, in agreement with the genotype and phenotype of each tumour. These results show that, in EBV+ nasal lymphomas of NK, T- or B-cell lineage, EBV was consistently associated with the tumour-cell population and support the view that EBV serves a promoting role in the pathogenesis of different types of EBV+ nasal lymphoma. © 1995 Wiley-Liss, Inc.