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Background The lack of specific markers for the phenotyping of circulating neoplastic T cells in Sézary syndrome (SS) patients makes it difficult both to ascertain the presence of clonal cells and to quantify the tumour burden in the peripheral blood. In previous reports we showed that the lack of CD26 (dipeptidyl-aminopeptidase IV) is a characteristic feature of circulating Sézary cells (SC). Objectives The purpose of this study was to ascertain, by means of high-resolution two-, three- or four-parameter flow cytometry, the relationship between CD26 expression on peripheral blood lymphocytes and peripheral blood involvement in cutaneous T-cell lymphoma patients and to assess its significance in SS diagnosis. Methods The patient population included 52 SS patients, 151 mycosis fungoides (MF) patients at different clinical stages (including 14 with blood involvement, B1-MF), 88 patients with erythrodermic inflammatory skin diseases (EISD) and 72 healthy donors (HD). CD26+ values were available in all cases, whereas CD4+ CD26– level measurement was performed in 23 SS, 141 MF, 71 EISD and 72 HD. Results CD4+ CD26– percentage values were higher than 30% in all but one B1-MF and higher than 40% in all SS cases, whereas HD, EISD and B0-MF patient values were always lower than 30%. A statistically significant difference was found in both CD26– and CD4+ CD26– percentage and absolute values between SS and HD, EISD and B0-MF patients. The CD26– and CD4+ CD26– percentage values (but not the absolute values) were significantly higher in B1-MF compared with HD, EISD and B0-MF patients (P < 0·001). Moreover, CD26– absolute values and CD4+ CD26– percentage and absolute values were significantly higher in SS than in B1-MF (P < 0·001). A statistically significant direct relationship was found between CD4+ CD26– percentage... [ABSTRACT FROM AUTHOR]