Antibody levels to measles were evaluated in 16 HIV-seropositive and 34 seronegative pregnant women and in cord blood samples of their infants. Infants were followed prospectively, and a second blood sample was taken at 3-7 months of age. Antibody levels were significantly lower in HIV-seropositive pregnant women and HIV-exposed cord bloods at birth (p=0.01 and 0.04, respectively) compared to controls. The prevalence of protective immunity (> or =1.09 optical density ratio) was also significantly lower (p=0.02) in HIV-seropositive pregnant women. T-cell counts were lower in HIV-seropositive women who were nonimmune (268/mm3) to measles compared to those who were immune (618/mm3), but the difference did not reach significance (p=0.07). Immunity to measles declined significantly to nonprotective levels in all infant samples obtained at 3-7 months of age. A secondary analysis determined the impact of mother's place of birth. Antibody levels were significantly higher (p=0.03) in foreign-born HIV-seronegative pregnant women and cord blood samples (p=0.01) compared to U.S.-born HIV-seronegative pregnant women seen in our inner-city clinic. Thus, HIV-seropositive and even some seronegative U.S.-born women may need a booster vaccine to ensure passage of adequate levels of passive immunity.