Highly active antiretroviral therapy (HAART) has been shown to be highly effective in reducing plasma levels of HIV RNA; therefore, these treatments could diminish the risk of transmission. We analyzed 393 steady heterosexual couples, of which one partner had been previously diagnosed with HIV infection (index case) and where the nonindex partner reported his or her sexual relationship with the index case as the unique risk exposure. These couples were consecutively enrolled in the period 1991 through 2003 when the nonindex partners took their first HIV test. HIV prevalence among partners of index cases who had not received antiretroviral therapy was 8.6%, whereas no partner was infected in couples in which the index case had been treated with HAART (P = 0.0123). HIV prevalence among nonindex partners declined from 10.3% during the pre-HAART period (1991-1995) to 1.9% during the late HAART period (1999-2003; P = 0.0061). In the multivariate analysis, this decline held (odds ratio = 0.14, 95% confidence interval: 0.03-0.66) after adjusting for length of partnership, unprotected coitus, and pregnancies as well as gender, CD4 lymphocyte count, AIDS-defining diseases, and sexually transmitted infections in the index case. When HAART became widely available, a reduction of approximately 80% in heterosexual transmission of HIV was observed, irrespective of changes in other factors that affect transmission.