Interventions are unlikely to achieve their desired aims unless they are implemented as intended. This paper focuses on factors that impeded or facilitated the implementation of a specially designed sex education programme, SHARE, which 13 Scottish schools were allocated to deliver in a randomized trial. Drawing on qualitative and quantitative data provided by teachers, we describe how this intervention was not fully implemented by all teachers or in all schools. Fidelity to the programme was aided by intensive teacher training, compatibility with existing Personal and Social Education (PSE) provision, and senior management support. It was hindered by competition for curriculum time, brevity of lessons, low priority accorded to PSE by senior management, particularly in relation to timetabling, and teachers' limited experience and ability in use of role-play. The nature of the adoption process, staff absence and turnover, theoretical understanding of the package, and commitment to the research were also factors influencing the extent of implementation across and within schools. The lessons learned may be useful for those involved in designing and/or implementing other teacher-delivered school-based health promotion initiatives.