There is a need for effective prevention programmes aimed at reducing risk behaviour among South African adolescents. HealthWise South Africa is a school-based programme designed to reduce sexual and substance use risk behaviour, and promote positive use of leisure time among high-school learners (students). Based on successful programmes in the United States of America, HealthWise was developed for use in South Africa and pilot tested in four South African high schools. We carried out a process evaluation to establish the fidelity of implementation and make sure HealthWise was culturally relevant. Data sources comprised focus groups with educators and learners, lesson evaluations and observations, and interviews with school principals. Qualitative analysis of data highlighted pertinent cultural and contextual factors and identified areas for modifying HealthWise in order to promote better programme-consumer fit. These areas centred on time, language, and leisure. We noted a dynamic tension between the educators' desire to adhere to plan, and to make adaptations in accordance with learners' needs and the context. Ultimately, researchers need to find a balance between fidelity of implementation and programme adaptation to obtain effective programmes that are culturally acceptable to local consumers. (Contains 2 tables.)