Objectives. Young people with inflammatory arthritis can have severe disease warranting biologic therapy. They face complex treatment decisions, with profound consequences. This study aimed to explore the influence of individuals outside the care team (trusted others) on the treatment decisions made by young people, in particular their decisions about biologic therapies. Methods. Young people (16-25 years of age) with inflammatory arthritis and experience of treatment decision making were recruited from three NHS Hospital Trusts. Twenty-five were interviewed, plus 11 trusted others identified by young people as being involved in their decision making, as well as 6 health professionals. The data were analysed using coding, memoing and mapping techniques and the findings were tested through a series of focus groups. Results. Young people initially emphasized their decisional autonomy, typically describing people other than health professionals as limited in influence. However, discussions revealed the involvement-in deliberation and enactment-of a range of other people. This cast of trusted others was small and largely consistent; mothers played a particularly prominent role, providing cognitive, practical and emotional support. Members of the wider cast of trusted others were involved in more limited but still significant ways. Conclusion. Young people claim autonomy but other people enable this. The network of relationships in which they are embedded is distinctive and evolving. Mothers play a supporting role well into early adulthood; in contrast, partners are involved in far more limited ways. As such, the applicability of adult models of decision making is unclear. This must be taken into account if the support provided by professionals is to be optimally tailored to young people's needs.