The treatment of childhood leukaemia is seen as a successful historical example of the operation of the randomized controlled trial and continues to inform contemporary policy making on such trials within health research. This article analyses the scientists' 'story of success' through historical research. It tells us about the organizational and professional structures of such research post-war in the United Kingdom, and examines the history of the cancer clinical trial through this particular example. The story reveals a more complex picture than the 'heroic' one, with key developments in the operation of post-war science, both in terms of its infrastructure and of its scientific networks, not least the rise of co-operative working among clinicians and the growing importance of statisticians in medical research and practice. It also underlines differences between the British and US approaches in which the role of one health system, the National Health Service, helped structure different, initially less intensive, patterns of response.