Background: Despite a global decline in mortality and morbidity from measles in the last decade, outbreaks continue to occur in some parts of the world including Nigeria.Objective: To determine antibody response to routine measles vaccinationin Nigerian children and evaluate vaccine potency.Methods: A prospective study of 234 children selected from 3 health centres in an urban area of Lagos, Nigeria. Blood was obtained before and 8-12 weeks after routine vaccination with Edmonston-Zagreb strain of measles vaccine. Antibodies were detected using the measles antibody neutralization test. Reconstituted vaccines samples were analysed forpotency on monolayers of Vero slam cells in 96-well tissue culture plates.Results: Twenty seven(11.5%) had pre-vaccination antibodies. Seroconversion rate among the 195 who returned for postvaccinationsampling was 69.2%: It was however 74.2% in children with no pre-vaccination antibodies compared to 17.6% in those with antibodies. (p<0.05). Only six (50%) of the measles vaccine vials were potent. Seroconversion rate among subjects vaccinated from potent vials was 74.3% compared with 42.9% in those vaccinated from non-potent vials (p = 0.006).Conclusion: Seroconversion to measles vaccination in our environmentis sub-optimal, partly attributable to low vaccine potency. Improvement of vaccine handling processes and booster doses of the vaccine are recommended.