Combined immunofluorescence (IF) and fluorescence in situ hybridization (FISH) on formalin-fixed, paraffin-embedded tissue sections were used to examine lymph node tissue from two patients diagnosed with T-cell lymphoma with Epstein-Barr virus (EBV)-infected B-cell blasts. The majority of cells within the samples comprised T-cells staining positively for CD3. In addition, both patients had a population of large pleiomorphic cells that were positive for the B-cell marker CD20 and for EBV LMP-1. Standard PCR clonality testing of the nodes revealed both immunoglobulin heavy chain (IGH) and T-cell receptor (TCR) clonal rearrangements in one patient, although in the other case monoclonality was demonstrated only for TCRG. Cytogenetics of cultured lymphocytes from nodal tissue revealed two apparently unrelated abnormal clones in both patients. Combined IF and FISH revealed that these phenomena reflected two abnormal populations of B- and T-cells rather than reactive B-cell hyperplasia or biphenotypic evolution from a common ancestral lymphoma. True B-cell malignancy probably emerged within a preexisting but unrelated T-cell lymphoma. This is the first study to relate the phenotype of the abnormal cells in such cases to specific clonal populations of cells, and it demonstrates a method that may easily be introduced into a diagnostic cytogenetics laboratory with access to standard pathology laboratory resources.