Falls in levels of measles, mumps, and rubella (MMR) immunisation in the UK and the continuing debate on how to respond to this situation emphasise the importance of identifying and understanding the factors that affect the uptake of recommended childhood immunisations. Both qualitative and quantitative evidence could be useful in this process. We aimed to explore the feasibility and value of an approach to formal synthesis of qualitative and quantitative evidence in the context of factors affecting the uptake of childhood immunisation in developed countries. We used a Bayesian approach to meta-analysis. Evidence from 11 qualitative and 32 quantitative studies of factors affecting uptake of childhood immunisation was combined and assessed. We conclude that use of either qualitative or quantitative research alone might not identify all relevant factors, or might result in inappropriate judgments about their importance, and could thus lead to inappropriate formulation of evidence-based policy. Further development of our methods might enable rigorous synthesis of qualitative and quantitative evidence in this and other contexts.