Objective To provide evidence about the sexual health knowledge, attitudes and behaviours of an ethnically diverse sample of young people from Secondary/High schools in London. Design Cross-sectional questionnaire-based survey. The sample consisted of students in school Years 11 to 13 (aged 15–18 years), present in school on the day of questionnaire administration. Setting Sixteen Secondary/High schools in London where the Black and Minority Ethnic (BME) population exceeded two-thirds of the intake. Questionnaires were completed by 2602 students aged 15 to 18, who self-defined their ethnicity as follows: White British ( n= 559), White Other ( n = 256), Black ( n = 710) and Asian ( n = 1077). Method Self-administered 30-minute questionnaire completed under ‘exam’ conditions, distributed and collected by a team of ethnically diverse fieldworkers. This questionnaire recorded socio-demographic information, sexual health knowledge, sexual health attitudes, experience of sexual intercourse, and sexual risk behaviour and outcomes. Results BME groups, relative to White British, generally reported poorer sexual health knowledge. Low level knowledge regarding STIs was particularly evident. The White British students were generally the most liberal in their attitudes to sex. With 65 per cent of Black Caribbean males reporting experience of sexual intercourse, and 48 per cent reporting first intercourse under the age of 16, this group is possibly the most at risk of poor sexual health. Although less likely to report sexual inter-course than Black Caribbeans, Black African females and Asian males and females also reported risk behaviour. Conclusion The findings demonstrate diverse sexual health knowledge, sexual attitudes and sexual behaviours among ethnic groups. This diversity demonstrates the importance of tailoring health education and promotion to meet the specific needs of ethnic groups.