Administration of 100000 IU vitamin A at the time of measles immunisation is currently recommended for infants in developing countries. However, the safety and value of giving vitamin A, a potent immune enhancer, with live measles virus vaccines are unknown. We conducted a randomised, double-blind, placebo-controlled clinical trial in Indonesia to evaluate the effect of simultaneous vitamin A supplementation on the immune response to measles immunisation at six months of age. 336 infants received either vitamin A (100000 IU) or placebo when immunised with standard-titre Schwarz measles vaccine. 82% of infants seroconverted to measles. In a multiple logistic regression model adjusting for maternal antibody titres, vitamin A supplementation was associated with a lower likelihood of seroconversion to measles (odds ratio 0.40, 95% Cl 0.19-0.88), and girls were less likely to seroconvert than boys (0.34, 0.15-0.76). Immunisation with standard-titre Schwarz vaccine at six months of age in this study population is characterised by high seroconversion rates. However, simultaneous high-dose vitamin A may interfere with seroconversion to live measles vaccine in infants with maternal antibody.