BACKGROUND: The age at which passively acquired antibodies are lost is critical to determining the optimal age for measles vaccination. Little is known about the influence of human immunodeficiency virus type 1 (HIV-1) infection on levels of prevaccination antibodies to measles virus. METHODS: Antibodies to measles virus were measured by plaque reduction neutralization assay in HIV-1-infected, HIV-seropositive but uninfected, and HIV-seronegative Zambian infants aged 6 weeks to 9 months. Regression models were used to estimate age-specific antibody concentrations. RESULTS: Neutralizing antibodies to measles virus were measured in 652 plasma samples collected from 448 infants, of whom 61 (13.6%) were HIV-1 infected, 239 (53.4%) were HIV seropositive but uninfected, and 148 (33%) were HIV seronegative. The best fitting model suggests that HIV-1-infected infants have lower levels of passively acquired antibodies to measles virus at birth than do HIV-seronegative infants, but their antibody levels decrease more slowly. By 6 months of age, 91% (95% confidence interval, 83%-99%) of HIV-1-infected infants, 83% (95% confidence interval, 77%-89%) of HIV-seropositive but uninfected infants, and 58% (95% confidence interval, 51%-64%) of HIV-seronegative infants were estimated to have antibody levels that were unlikely to affect immune responses to measles vaccine (cutoff value for immune response, <50 mIU/mL). By 9 months of age, 99% of all infants had antibody levels <50 mIU/mL. CONCLUSIONS: Infants born to HIV-1-infected women are less likely to have passively acquired antibodies that would neutralize measles vaccine virus and, thus, have an increased risk of measles prior to the age of routine vaccination. Protection could be achieved by administration of the first dose of measles vaccine prior to 9 months of age.