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The expansion of CD4+CD7- T cells in the peripheral blood of Sézary syndrome (SS) is well known. It remains unclear whether this population contains the dominant T cell clone. Peripheral blood mononuclear cells (PBMC) of five SS patients were sorted by fluorescence-activated cell sorting into CD4+CD7- and CD4+CD7+ populations. These populations were analysed separately for clonality of the T cell receptor gamma chain (TCR-gamma) by PCR-DGGE. The cytokine profile of both populations was investigated by RT-PCR ELISA for IFN-gamma, IL-2, IL-4, IL-5, IL-10, IL-13 and IL-15. In three other patients with known Vbeta-usage, the dominant T cell clones were phenotypically characterized by double staining. PCR-DGGE of TCR-gamma demonstrated that all patients had a clonal population in their blood and that this population was present in CD4+CD7- and CD4+CD7+ populations. Concerning mRNA cytokine transcription, the two populations did not show any consistent differences. In three patients with identified clones (Vbeta 3.1, 5.3 and 6.7), double staining revealed positivity for CD2, CD3, CD4, CD5, CD45RO and CD7 in a significant proportion (at least 35%). We conclude that the CD4+CD7- population does not represent the dominant T cell clone in patients with SS. An increase in this population of PBMC in SS might account for deviations in the T cell functions of the patients.