A large measles outbreak occurred in South Wales in 2012/2013. The outbreak has been attributed to low take-up of measles-mumps-rubella (MMR) immunization in the early 2000s. To understand better the factors that led to this outbreak we present the findings of a case-control study carried out in the outbreak area in 2001 to investigate parents' decision on whether to accept MMR. Parents who decided not to take-up MMR at the time were more likely to be older and better educated, more likely to report being influenced by newspapers [adjusted odds ratio (aOR) 3·07, 95% confidence interval (CI) 1·62–5·80], television (aOR 3·30, 95% CI 1·70–6·43), the internet (aOR 7·23, 3·26–16·06) and vaccine pressure groups (aOR 5·20, 95% CI 2·22–12·16), and less likely to be influenced by a health visitor (aOR 0·30, 95% CI 0·16–0·57). In this area of Wales, daily English-language regional newspapers, UK news programmes and the internet appeared to have a powerful negative influence. We consider the relevance of these findings to the epidemiology of the outbreak and the subsequent public health response.