This article explores the political history of school sex education policy in England and Wales. Focusing on the period from the 1940s to the 1960s, it shows how sex education developed as a controversial political issue through an analysis of the differing institutional cultures and agendas of health and education administrators. The article argues that serious consideration of school sex education by central government was first prompted by concern about venereal disease during the Second World War. Thereafter, two groups of actors emerged with conflicting ideas about the role of government in prescribing school sex education. The medical establishment, including the Ministry of Health, was broadly supportive of a national policy, whereas the Department of Education, which had ultimate responsibility for any such policy in schools, sought to avoid decision-making about the issue. The article explores how a public health consensus on sex education developed and then explains why the Department of Education resisted this consensus.