Adult T‐cell leukaemia/lymphoma (ATLL) is an aggressive neoplastic disease that usually exhibits a CD4⁺CD25⁺ phenotype. Regulatory T cells (Treg), which suppress T‐cell effector function, are characterized by the co‐expression of CD4 and CD25. We analysed the expression of forkhead/winged helix transcription factor (FoxP3), a specific marker that is important for the function of Treg, on ATLL cells from 17 patients (peripheral blood, n = 8; lymph node, n = 9). Real‐time polymerase chain reaction and immunostaining detected FoxP3 expression in 10 ATLL cases, but was relatively down‐regulated compared with Treg from normal subjects. These results indicate the association of ATLL and Treg.