We compared 224 heterosexual couples who were discordant for human immunodeficiency virus (HIV) type 1 infection (one partner HIV infected) with 78 HIV-concordant couples (both partners HIV infected) to identify demographic and behavioral risk factors for HIV transmission. Among the 229 couples whose male partner was first infected, HIV-concordant couples had engaged in anal sex more frequently before and after knowing that the male was infected than had HIV-discordant couples. Pap smears of grade 2 or higher (inflammation) were more prevalent among the secondinfected female partners in HIV-concordant couples than among uninfected women in discordant couples (58% vs. 23%; P < .001). Anal sex and unprotected vaginal sex after knowledge of a male partner's infection were significant correlates of concordance in a multivariate logistic model, as were ethnicity, marital status, and antiviral therapy. Ethnicity strongly predicted concordance, even after controlling for sexual risk behaviors and stage of disease.