We investigated seroconversion rates in febrile children after measles vaccination. Among 6364 vaccinees, 501 children had a temperature of 37.5°C or higher within 7 days of vaccination. The seroconversion rate assessed by a haemagglutination-inhibition assay was 76.6% in 501 febrile children but 95.2% in 84 afebrile controls.