Post-primary school students (n = 2407) and young adults (n = 477) participated in a cross-sectional evaluation of a health education programme for schools. The Lifeskills programme is based on a philosophy of student empowerment, and aims to teach knowledge and skills relevant to health promoting behaviour. School students were recruited in schools, while young adults were opportunistically recruited in workplaces, training centres and on public transport. Those who attended schools where Lifeskills had been taught and who remembered such lessons were conservatively classified as the intervention group, while those who attended other schools and did not remember such lessons were classified as the comparison group. Participants completed questionnaires designed to collect data on health-related behaviours, indicators, knowledge and psychological health. School-level factors were employed as covariates in subsequent analyses of covariance. Amongst younger pupils, females reported more positive health behaviours but lower levels of psychological well-being and more symptoms. The impact of the programme became evident at ages 13¿15. Those involved drank less and reported more positive adjustment to school. However, sex differences remained, with females reporting more health-promoting behaviour and more symptoms, and lagging behind males in self-esteem and general well-being. An interaction between gender and the intervention was identified among senior pupils. Exposure was especially beneficial for females. However, as young adults, the two main effects of gender and programme participation re-emerged as the most important independent variables, and the interaction between them was not significant. This pattern has implications for the interpretation of evaluations conducted on short-term interventions as well as for short-term impact evaluations.