Because of concern that steady sexual partners of patients infected with human immunodeficiency virus (HIV) may be infected despite negative results in tests for antibody to HIV, we studied 50 sexually active couples with discordant antibody results, assessing the agreement between these serological results and those obtained by p24 antigen testing, the polymerase chain reaction (PCR), and culture. Forty-nine of 50 seropositive sexual partners were also positive for HIV by PCR; the remaining seropositive partner was positive by culture. All seronegative partners also had negative results in the other three tests. Moreover, seronegative partners continued to have negative results in all tests for a mean follow-up period of 17 months despite ongoing sexual relations with their seropositive partners. Seronegative infection was not documented in these partners at risk for sexual transmission of HIV. HIV-negative individuals in stable, monogamous sexual relationships with HIV-infected partners apparently do not have a high incidence of infection despite continued sexual exposure.