Early childhood infection with human T cell lymphotropic virus type I (HTLV-I) has been suggested to be involved in the pathogenesis of infective dermatitis and adult T cell leukemia/lymphoma. Since only a very small percentage of HTLV-I-infected children develop disease later in life, identification of early interim markers for persons at risk for developing disease would enable monitoring and might provide insight into the pathophysiology of the various diseases associated with HTLV-I infection. A cross-sectional study analyzed T cell subsets in 35 HTLV-I-seronegative and 16 HTLV-I-seropositive Jamaican children 11-31 months old. HTLV-I seropositivity was associated with an increase in the mean percentage of CD4 cells expressing HLA-DR, a marker for T cell activation (P = .02). This increase was positively correlated with duration of infection (r = .74, P = .009). These data demonstrate perturbation of regulatory cells of the immune system in HTLV-I-infected children.