BackgroundDespite evidence showing that adolescent-friendly health services (AFSs) increase young people's access to these services, health systems across the world are failing to integrate this approach. In Latin America, policies aimed at strengthening AFS abound. However, such services are offered only in a limited number of sites, and providers’ attitudes and respect for confidentiality have not been addressed to a sufficient extent.MethodsThe aim of this study was to explore the mechanisms that triggered the transformation of an ‘ordinary’ health care facility into an AFS in Ecuador. For this purpose, a realist evaluation approach was used in order to analyse three well-functioning AFSs. Information was gathered at the national level and from each of the settings including: (i) statistical information and unpublished reports; (ii) in-depth interviews and focus group discussions with policy makers, health care providers, users and adolescents participating in youth organisations and (iii) observations at the health care facilities. Thematic analysis was carried out, driven by the realist evaluation approach, namely exploring the connections between mechanisms, contexts and outcomes.ResultsThe results highlighted that the development of the AFSs was mediated by four mechanisms: grounded self-confidence in trying new things, legitimacy, a transformative process and an integral approach to adolescents. Along this process, contextual factors at the national and institutional levels were further explored.ConclusionThe Ministry of Health of Ecuador, based on the New Guidelines for Comprehensive Care of Adolescent Health, has started the scaling up of AFSs. Our research points towards the need to recognise and incorporate these mechanisms as part of the implementation strategy from the very beginning of the process. Although contextually limited to Ecuador, many mechanisms and good practices in these AFS may be relevant to the Latin American setting and elsewhere.