After a report of reduced seroconversion to measles in infants, aged 6 mo, given vitamin A with their measles vaccination, serious concerns were raised regarding the safety of the WHO's recommendation that infants be supplemented with vitamin A at the time of measles immunization. To determine the impact of coadministered vitamin A on the antibody response to measles vaccine given to infants aged 9 mo, the more common age for immunization in developing countries, we conducted a randomized, double-blind, placebo-controlled trial in an urban slum community in Delhi. Infants (618) were randomly allocated to receive 30 mg vitamin A or a placebo with the measles immunization. Antibodies to measles were measured by ELISA in serum samples obtained at before (baseline) and 12 wk after immunization. Overall, the seroconversion rates did not differ between vitamin A (89.5%) and placebo (87.6%) groups. There were no significant differences in the geometric mean titers in the two groups (ratio of geometric means, 1.19; 95% confidence interval, 0.97-1.46). Among malnourished infants, the geometric mean titer was significantly greater in the vitamin A group compared to the placebo group (ratio of geometric means, 1.57; 95% confidence interval, 1. 18-2.0), but seroconversion rates did not differ. There were no differences in seroconversion rates and geometric mean titers in the two study groups among the well-nourished children. These results indicate that 30 mg vitamin A does not reduce the immune response to the coadministered vaccine and, therefore, can be continued to be given safely in public health programs.