Background Levels of measles in England and Wales are at their highest for 18 years, and strategies targeting the different groups of parents who do not vaccinate their children continue to be needed. Decision aids for decisions regarding childhood immunisation appear to be effective in achieving an increase in vaccine uptake but their cost effectiveness is unknown. Aim To assess the cost effectiveness of a web-based decision aid to increase uptake of the MMR vaccine. Design and setting Economic evaluation conducted alongside a cluster randomised controlled trial using urban GP practices in the north of England. Method Fifty GP practices in the north of England were randomised to one of three trial arms: decision aid, leaflet, usual practice. A total of 220 first-time parents (child aged 3-12 months) were recruited. Parents self-reported their contacts with the NHS and other previous/expected resource utilisation; associated costs were calculated. Vaccine-uptake data were collected from GP practices. A cost-effectiveness analysis was undertaken and provided the incremental cost per first-vaccine uptake. Multiple imputation was used to account for missing data and findings were adjusted for baseline differences in parents' levels of decisional conflict regarding MMR vaccination. Results Of the 220 first-time parents recruited to the study, 179 completed the baseline and postintervention questionnaires. MMR uptake was highest for those receiving the decision aid (42 out of 42, 100%) versus usual practice (61 out of 62, 98%) and leaflet arm (69 out of 75, 92%), and was associated with lower cost (-epsilon 9.20 versus usual practice and -epsilon 7.17 versus leaflet). Conclusion The decision aid has a high chance of being cost effective, regardless of the value placed on obtaining additional vaccinations. It also appears to offer an efficient means of decision support for parents.