Sialyl Lewis(x) antigen-positive (sLe(x+)) cells play an important role in the first step of transmigration of T lymphocytes through vascular endothelial cells into tissues. We compared the proportion of sLe(x+) cells in peripheral blood CD4(+) T lymphocytes between patients with HTLV-1-associated myelopathy (HAM) and control patients, by using flow cytometry. The percentage of sLe(x+) cells in peripheral blood CD4(+) T lymphocytes was significantly higher in HAM patients compared to control patients. Interferon-gamma (IFN-gamma), but not interleukin-4 (IL-4), production by the sLe(x+) cell population of peripheral blood CD4(+) T lymphocytes was significantly higher in HAM patients than in control patients. In addition, comparison of the HTLV-1 proviral load between sLe(x+) and sLe(x-) cells in peripheral blood CD4(+) T lymphocytes of HAM patients showed that the HTLV-1 proviral load was significantly higher (two- to eight-fold), concomitant with enhanced IFN-gamma production, in the sLe(x+) than in the sLe(x-) cell population. Our findings indicate that the sLe(x+) cell population, which has features of activated Th1 cells with up-regulated expression of E- and P-selectin ligands mediated by HTLV-1 infection, is increased in peripheral blood CD4(+) T lymphocytes in HAM patients, suggesting their involvement in transmigration of peripheral T lymphocytes from the peripheral blood into tissues.