Cutaneous T-Cell lymphoma (CTCL) is a non-Hodgkin's lymphoma of unknown pathogenesis. Mycosis fungoides (MF) is a clinically determined subset of CTCL with intensive infiltration of lymphoma cells into the epidermis. To determine whether Epstein-Barr virus (EBV) is associated with these lymphoma cells, we performed mRNA in situ hybridization in 5 cases of CTCL and 7 cases of MF using an RNA probe transcribed from BamHI W fragment of EBV genome. These transcripts were detected in the majority of lymphoma cells in all cases examined. We also detected intensive hybridization signals on epidermal squamous cells contiguous to strong infiltration with lymphoma cells into the subcutaneous connective tissue. Similarly, positive signals were detected using the probes transcribed from the sequences of EBV-encoded small nonpolyadenylated RNAs-1 (EBER1) and EBV-determined nuclear antigen-2 (EBNA2). The EBNA2 latent membrane protein-1 (LMP1) and BZLF1 product (ZEBRA) were also detected by immunofluorescence staining using monoclonal antibodies. Further in the same experiment, we detected immunofluorescence of epidermal cells. EBV DNA was detected in all cases tested by DNA in situ hybridization. Moreover, we also identified the signals on epidermal cells via this technique. Polymerase chain reaction revealed amplified EBV DNA for most cases tested. Double staining with immunohistochemistry and RNA in situ hybridization showed that T-cell marker-positive cells, but not EBV-carrying B-cells, exhibited signals for the EB viral RNA. These findings suggest that EBV is involved in the neoplastic transformation of CTCL and MF.