Three possible reasons for failure of measles vaccine were identified as follows: (a) circulating maternal antibodies, (b) administration of non-viable vaccine, and (c) misdiagnosis. A study designed to test the role played by each of these in the high measles vaccine failure rate was carried out at the Oguntolu Street Clinic, Lagos, from October 1982 to February 1983. Failure to seroconvert was the cause of 67.7% of measles vaccine failure, particularly at ages 6-7 months, though this was not associated with high prevaccination titres. In 32.3% incorrect diagnosis was responsible. Non-viable vaccine was found not to be a reason because all vials of measles vaccine were potent at the time of administration.