During March to May 2006 the highest incidence of measles in New South Wales since 1998 provided an opportunity to estimate the effectiveness of the measles-mumps-rubella (MMR) vaccination program in preventing childhood measles, and describe any differences in clinical presentation between vaccinated and unvaccinated children. We reviewed records of all 33 notified cases of measles in children aged 1-14 years during a state-wide outbreak in New South Wales from March - May 2006. Six of the children had a confirmed history of vaccination with at least 1 dose of MMR. The children with previous vaccination tended to have milder disease than those without vaccination as judged by their reported number of symptoms and hospitalisation rates. The vaccinated children were less likely to have a typical measles rash. Two of the cases in previously vaccinated children may be due to secondary vaccine failure, although a lack of complete diagnostic testing limits our ability to confirm this. Vaccine effectiveness after receiving at least 1 dose of MMR is estimated to be 96% (95% CI 77.8-99%). MMR vaccination was effective in preventing measles in children during these outbreaks.