OBJECTIVES: A Gateway to Medicine programme, developed in partnership between a further and higher education setting and implemented to increase the socioeconomic diversity of medicine, was examined to identify precisely what works within the programme and why. DESIGN: This study employed realist evaluation principles and was undertaken in three phases: document analysis and qualitative focus groups with widening access (WA) programme architects; focus groups and interviews with staff and students; generation of an idea of what works. SETTING: Participants were recruited from a further/higher education setting and were either enrolled or involved in the delivery of a Gateway to Medicine programme. PARTICIPANTS: Twelve staff were interviewed either individually (n=3) or in one of three group interviews. Nine focus groups (ranging from 5 to 18 participants in each focus group) were carried out with Gateway students from three consecutive cohorts at 2-3 points in their Gateway programme year. RESULTS: Data were generated to determine what 'works' in the Gateway programme. Turning a realist lens on the data identified six inter-relating mechanisms which helped students see medicine as attainable and achievable and prepared them for the transition to medical school. These were academic confidence (M1); developing professional identity (M2); financial support/security (M3); supportive relationships with staff (M4) and peers (M5); and establishing a sense of belonging as a university student (M6). CONCLUSIONS: By unpacking the 'black box' of a Gateway programme through realist evaluation, we have shown that such programmes are not solely about providing knowledge and skills but are rather much more complex in respect to how they work. Further work is needed to further test the mechanisms identified in our study in other contexts for theory development and to identify predictors of effectiveness in terms of students' preparedness to transition.