The advocates of Vaccination have exalted in the prospect of exterminating the Small Pox from the face of the earth; while its opponents have framed their tales of horror, replete with stories of novel diseases and unheard-of plagues. ... When I consider the many evidences in favour of Vaccination, which the public documents of almost every nation afford, I am at a loss to conceive from what cause such doubts can have arisen; for I think, if an unprejudiced mind will fairly consider the question, it must be convinced that Vaccination has answered the promised end. (from a letter to The Gentleman's Magazine , March 1808, by “Cosmopoliton”) Since the introduction of the measles vaccine in 1968 and then the measles, mumps, and rubella (MMR) vaccine in 1988, measles, mumps, and rubella have become rare diseases. However, on the basis of research, primarily from one unit, some parents and health professionals have become concerned about the safety of MMR vaccine. This has led to a decline in uptake and fears of impending outbreaks.1 In this paper we review the background to these concerns and examine the evidence to date. In the early 1990s the Inflammatory Bowel Disease Study Group (IBDSG) at the Royal Free Hospital in London suggested that measles disease in children predisposed them to inflammatory bowel disease as adults.2 3 However, further research, including that by the IBDSG has shown this not to be so.4-6 Later, the IBDSG suggested that measles vaccination could lead to adult inflammatory bowel disease.7 This study was much criticised at the time8 and further work, including some by the IBDSG, has shown no link.9 10 More recently, Ghosh et al reviewed all the evidence surrounding the presence of measles virus in Crohn's disease and concluded …