Background: The study aimed to examine the impact of household composition and satisfaction with family life on sexual behaviours among high school male and female students (aged 11-22 years) in Hong Kong.; Method: High schools were randomly selected, and the final sample comprised 25 schools. Students were divided into two groups ('living with both biological parents' vs 'not living with both biological parents'). Students were asked to rate their satisfaction with family life on a five-point Likert scale in a self-administered questionnaire. Dependent variables were sexual experience, sexual harassment, sexting and nude chats. Multiple logistic regression was used to analyse the results.; Results: 3907 students were included in the analysis. 202 students (5.2%) were sexually active. 505 students had ever (13.0%) sexually harassed others and 303 students (7.8%) had ever been sexually harassed by others. 58 students (1.5%) had ever had nude chats. 1005 students (25.8%) had sexted in the last 12 months. Students who lived with both biological parents were less like to be sexually active, to sext and to have nude chats than those who did not. Students who had higher family life satisfaction were less likely to be sexually active, to sexually harass others, to be sexually harassed by others, to sext and to have nude chats than students who had lower satisfaction with their family life.; Conclusions: Sexual health programmes and interventions should consider family functioning. Students who have low family satisfaction and those who do not live with both their biological parents should be targeted for sexual health interventions. [Copyright of BMJ Sexual & Reproductive Health is the property of BMJ Group.]