We investigated knowledge, attitudes, norms, and values of teenagers, parents, teachers, and policymakers in relation to sex and sex education in Chiang Mai, Thailand, with a view to informing sex education policy. Methods were: narrative interviews with key stakeholders, and analysis of key policy documents; questionnaire survey of teenagers; 20 focus groups of teenagers; questionnaire survey of parents; and two focus groups of parents. Qualitative and quantitative data were assessed separately with thematic and statistical analysis, respectively, then combined. FINDINGS: We noted five important influences on Thai teenagers' sexual attitudes and behaviour: ambiguous social roles leading to confused identity; heightened sexual awareness and curiosity; key gaps in knowledge and life skills; limited parental input; and impulsivity and risk-taking. Male teenagers aspire to be "smart boys", whose status depends on stories of sexual performance and conquests.