Data on T-cell subsets from 89 human T-cell lymphotropic virus-I (HTLV-I) carriers and 25 seronegative people were analyzed to identify differences in T-cell subset values among three subgroups: HTLV-I carriers with abnormal lymphocytes (Ably; n = 24), carriers without Ably (n = 65), and HTLV-I seronegatives (n = 25). Estimates of mean values were adjusted for age, sex, smoking, and alcohol drinking, as appropriate. The percentage of CD25+T cells was elevated in carriers with Ably (mean, 16.7 ± 1.0) compared with the seronegatives (11.4 ± 1.4;p= 0.0002); individuals with CD25 T-cell percentages above the median for the seronegatives had a corresponding 5.4-fold risk for being a carrier with Ably. Similarly, the percentage of CD4 T cells was elevated in carriers with Ably. Conversely, the percentage of CD8 T cells was lower among both groups of HTLV-I carriers than in the seronegatives. There was a corresponding significant increase (p= 0.0004) of the CD4/CD8 ratio among carriers with Ably (1.57 ± 0.12) compared with the seronegatives (1.22 ± 0.12). Among subjects with CD4/CD8 ratios above the median for the seronegatives, there were 6.8− and 4.5-fold risks for being carriers with or without Ably, respectively. The percentage of CD7 was lower among carriers with Ably (75.6 ± 1.6) than among seronegatives (78.9 ± 1.5;p= 0.13). The percentage of β-interleukin-2-receptor-positive T cells did not vary among the three subgroups. The results indicate that levels of CD25 and CD4 cells are increased among HTLV-I carriers, with the largest increase seen in carriers with Ably. Aside from changes in T-cell percentages due to clonal expansion of infected cells, altered cytokine production in response to HTLV-I infection may mediate changes in T-cell populations.