BACKGROUND Maternally derived passive measles antibody may interfere with vaccine-induced immunity in infants less than 12 months of age. However, early loss of passive measles antibody may occur in infants of women who received measles vaccine because measles vaccine induces lower antibody titers than does natural infection. METHODS Persistence of passive neutralizing measles antibody was studied longitudinally in a group of normal infants as a function of maternal measles titer at birth and maternal date of birth. Maternal serum and cord blood specimens were tested from 162 women and their newborns, from 51 of these infants at 9 months of age and from 63 at 12 months of age. RESULTS Seventy-one percent of sera from 9-month-old infants (36 of 51, 95% confidence interval 68% to 84%) and 95% of samples from 12-month-old infants (60 of 63, 95% confidence interval 89% to 101%) had no detectable neutralizing measles antibody. Measles geometric mean titers were significantly higher at delivery in mothers whose infants were seropositive at 9 and 12 months compared with mothers whose infants were seronegative at 9 and 12 months. All infants with detectable measles antibody at 9 or 12 months had mothers born before 1963, before the vaccine era, and both material and cord blood measles geometric mean titers decreased significantly with decreasing maternal age. CONCLUSIONS Persistence of passive measles antibody is uncommon by 12 months of age; earlier antibody loss is related to lower maternal age and maternal measles titer.