The majority of natural history studies of human immunodeficiency virus (HIV) infection have immune and viral parameters in men. Data demonstrating that women have lower HIV-1 RNA levels than men at the same CD4 cell counts have raised the question of immunologic differences in HIV-seropositive women. This study describes levels and changes in phenotypic markers of immune maturity, function, and activation in the CD4 and CD8 cell subsets in HIV-seropositive and high-risk HIV-seronegative women. Our primary hypothesis was that activation levels would be significantly higher among illicit drug users. However, results showed that HIV-1 RNA level was the strongest predictor of marker level and that both HIV-1 RNA level and CD4 cell count were independently associated with CD4 activation, but illicit drug use was not. In summary, this study demonstrated that immune activation was a significant pathogenic feature in women and that activation was driven by HIV infection and not illicit drug use.