BACKGROUND: Certain Black and Minority Ethnic (BME) groups have been identified as bearing a disproportionate burden of sexual ill health in the UK. There still exists a significant gap in the evidence-base concerning the sexual behaviours of BME youth. METHODS: We conducted a cross-sectional survey of 16 secondary schools in London, where the proportion of BME students exceeded two-thirds of the intake. The aim was to identify the factors associated with sexual intercourse and sexual risk behaviours. RESULTS: 2801 students aged 15 to 18 completed questionnaires. Compared with White British males, Black males were significantly more likely to report sexual intercourse (OR 3.04, 95% CI 1.92-4.81). Compared with White British females, Asian females were significantly less likely to report sexual intercourse (OR 0.43, 95% CI 0.27-0.69). Both sexes reporting school as their main source of information, as opposed to their family, had a lower likelihood of intercourse (males OR 0.33, 95% CI 0.21-0.53, females OR 0.39, 95% CI 0.25-0.62). Compared with White British males, contraceptive use at first ever intercourse was significantly less likely among Black males (OR 0.35, 95% CI 0.16-0.98) and males of Mixed ethnicity (OR 0.24, 95% CI 0.06-0.92). Among females, higher sexual health knowledge was associated with contraceptive use (OR 4.69, 95% CI 1.76-12.49). Males reporting higher sexual health knowledge were more likely to be 'sexually competent' at first ever intercourse (OR 2.28, 95% CI 1.10-4.69). CONCLUSIONS: Given the importance of sexual health knowledge, ensuring this is improved among all ethnic groups, especially the Black and Mixed male groups, is a key message to arise from this study. Delivering this information in a 'culturally competent' manner is an important goal.