Article published in Social Science & Medicine, Volume 71, Issue 6, Pages 1049-1055, September 2010. Based on a case-study of the introduction of measles-mumps-rubella (MMR) vaccine in the Netherlands two decades ago, using documentary and archival sources, this paper examines the way evidence is used in policymaking. Starting from the question of ‘what counts as evidence’, two central claims are developed. First, the decision to introduce MMR was not one but a series of decisions going back at least seven years, over the course of which the significance attached to various forms of evidence changed. Second, results of international studies were coming gradually to be of greater significance than evidence gathered from within the Netherlands itself. These developments had, and continue to have, major consequences for national scientific competences.