The Foxp3 protein is a specific marker of CD4(+)CD25(+) regulatory T (T(reg)) cells, and its expression is critical to their development and function. Several studies have demonstrated the dysregulation of Foxp3 expression during human inflammatory diseases. Infection with human T lymphotropic virus type 1 (HTLV-1) is associated with the development of a number of inflammatory conditions, including myelopathy, although the majority of individuals who are infected with HTLV-1 remain asymptomatic. To examine the role played by T(reg) cells in the development of inflammatory disease during HTLV-1 infection, we examined Foxp3 expression by flow cytometry. Our analysis showed that HTLV-1-associated myelopathy/tropical spastic paraparesis was associated with a lower expression (compared with that in asymptomatic HTLV-1 carriers and healthy donors) of Foxp3 in peripheral-blood leukocytes. In individuals infected with HTLV-1, Foxp3 expression was inversely correlated with HTLV-1 tax proviral DNA load. These results suggest that impaired Foxp3 expression may contribute to the development of inflammatory disease during HTLV-1 infection.