Copyright © 2019. Published by Elsevier Ltd. INTRODUCTION: Young people living with a chronic illness must be supported by healthcare professionals and parents to manage their care as they move through adolescence and into adulthood. The concept of agency is relevant to this process as it involves reflecting on one's behaviour and its consequences, and considering alternative possibilities, before committing to a course of action. METHODS: The aim of this study was to explore the role of agency in young people's transition to self-management of cystic fibrosis. Qualitative semi-structured interviews were carried out with 13 young people (7 female, 6 male) aged 13-22 years, recruited from three cystic fibrosis clinics in the Republic of Ireland. RESULTS: By self-monitoring their behaviour and managing their symptoms the young people described a process by which they employed agency and as a result, gained confidence and a sense of control over their illness. Although, parents and health care professionals aided in the development of agency, the young people explained that it was occasionally threatened, particularly in their interactions with health care professionals. This led the young person to react defensively, and negatively affected their relationship with health care professionals. CONCLUSIONS: For young people to feel supported empowered in the management of their illness, their developing ability to exercise agency must be respected and encouraged through collaborative decision making.