The purpose of this study was to measure the accuracy of HIV-infected persons' knowledge of their sexual partners' HIV infection status. HIV-infected persons at two public health clinics reported their knowledge of the HIV infection status of their sexual partners. Actual HIV test results for sexual partners were then linked to survey responses. The association between reported knowledge and actual infection status of partners was estimated by kappa scores and percent agreement. Sixty-four percent (14/22) of partners thought to be infected were actually uninfected, and 42% (8/19) of partners thought to be uninfected were actually infected. The overall percent agreement was 46% (kappa = -0.06), less than that expected by random chance alone. Individuals in committed partnerships were less likely to have accurate knowledge than those in casual partnerships. In conclusion, HIV-infected individuals have poor knowledge regarding their partners' infection status. This may influence sexual behaviors that result in increased transmission.