Objective: To assess the sexual health knowledge of teachers who contribute to secondary school sexual health education in order to determine whether teachers are adequately prepared to implement present government education and public health policies. Design: Results were obtained from a questionnaire as part of a two-phase intervention study. Setting: Nineteen mixed-sex, state secondary schools in central England. Participants: One hundred and fifty-five teachers (94 female, 61 male) participated. Main outcome measures: The questionnaires were distributed to teachers to assess their knowledge of sexual health, contraception and sexually transmitted infections. In addition, teachers' attitudes on the subject of sex and relationships education were evaluated. Results: The results suggest that teachers have insufficient sexual health knowledge to effectively teach sexually transmitted infections or emergency contraception, although their general sexual health knowledge was good. Therefore, at present teachers do not have adequate specialist knowledge in sexual health to contribute to current recommendations for sex and relationships education in secondary schools. There were no statistically significant differences in the results regarding location of school, area of residence, gender or age of the participant. Conclusions: Many teachers are being expected to contribute to secondary school sexual health education programmes at a time when they do not have sufficient knowledge to provide young people with adequate sexual health education and when they do not feel prepared to teach, and in many cases would prefer not to teach, these programmes. (Contains 4 tables and 3 figures.)