We used two-color flow cytometric analysis to investigate CD26+ (Ta1+) cells in peripheral blood T lymphocytes from patients with human T-lymphotropic virus type-I (HTLV-I)-associated myelopathy (HAM). The percentage of CD26+ cells among CD3+ cells was markedly increased in patients with HAM, compared with anti-HTLV-I seropositive carriers (p < 0.001) and seronegative controls (p < 0.01). Within the subpopulation of T cells, a significantly high percentage of CD26+ cells was detected in both CD4+ and CD8+ cell populations. Furthermore, analysis of HLA-DR+ T cells revealed similar results. In contrast CD4+CD45RA+ cells were significantly decreased in comparison with controls. These results suggest that immunologically activated or memory T cells found in peripheral blood may be etiologically relevant to HAM.