Objective: To assess what kinds of knowledge policymakers in a sample of English primary schools utilised to make decisions about their school's sex and relationships education policy. Method: Semi-structured interviews were conducted with policymakers at three primary schools in the southwest of England, and documentary analysis of the schools' sex and relationships education policies was done. Policymakers included head teachers, subject leads and other teachers. A social constructivist approach informed the research design and data analysis. Results: In all three schools, the principal sources of knowledge in the policy-making processes for sex and relationships education were a commercially available curriculum package and staff knowledge (including staff opinions). All three schools drew on policies from other schools, but at times this process appeared procedural rather than critical. Local authority expertise was a source of knowledge for two schools. Conclusion: Findings emphasise the importance of having trained and experienced educators for sex and relationships education in schools and the need for schools to have ongoing support to share learning and identify new knowledge on the topic.