Adult T-cell leukemia/lymphoma (ATL) is a CD4+CD25+ T-cell malignancy infected with human T-cell leukemia virus type-I (HTLV-I). HTLV-I infection causes the T-cell dysfunction, which contributes to the immunodeficient state of the patients. Programmed death-1 (PD-1) can negatively regulate T-cell response, when its ligand, PD-L1 or PD-L2 mainly expressed on antigen presenting cells, binds to this B7 family receptor. We investigated whether PD-1 is expressed on CD4+ neoplastic (and/or non-neoplastic) cells or CD8+ cytotoxic cells in peripheral blood mononuclear cells from 11 patients with ATL. By flow cytometry, we found that the levels of PD-1 expression on both CD4+CD25+ and CD4+CD25− T-cell populations were increased in ATL patients compared to normal healthy volunteers, while PD-1 levels on CD8+ T-cells were comparable between the patients and normal subjects. In stimulation with anti-CD3 antibody, the proliferation of PD-1-expressing T-cells from ATL patients was weak when compared to that of PD-1-nonexpressing normal T-cells. In addition to PD-1, PD-L1 was coexpressed on ATL cells in some patients, and PD-L1 expression was enhanced by stimulation with anti-CD3 antibody. Finally, the production of cytokines such as TNF-α by ATL cells was restored by blockade of PD-1/PD-L1 interaction. These findings suggest that CD4+ T-cells are the main PD-1-expressing cells rather than CD8+ T-cells in ATL patients, and both neoplastic and normal CD4+ cells are exhausted as a result of PD-1 expression, and additionally PD-L1 expression on the neoplastic cell. © 2007 Wiley-Liss, Inc.