Sex-based differences in the levels of human immunodeficiency virus 1 (HIV-1) RNA in plasma could be associated with differences in the strength of HIV-1-specific CD8(+) T cell responses. CD8(+) effector responses in 18 men and 15 women were measured 0-2 years (time A) and 5-7 years (time B) after seroconversion. CD8(+) effector responses were seen in 7 (39%) of 18 men and 2 (13%) of 15 women at time A (P=.13) and in 12 (67%) of 18 men and 10 (67%) of 15 women at time B (P=.99). At time B, the strength of CD8(+) effector responses correlated with the number of CD4(+) lymphocytes in women ( rho =-0.68; P=.005) but not in men ( rho =-0.14; P=.58). The level of HIV-1 RNA was not associated with the strength of CD8(+) effector responses according to sex, but there was a sex-based difference in the correlation between the strength of CD8(+) effector response and the number of CD4(+) lymphocytes.