BACKGROUND: While a growing body of research has explored why people take part in clinical trials, this research has not considered how people's understandings, motivations and agendas might influence their conduct during a trial. This is an important area of enquiry because it is now widely recognised that an intervention might lead to different clinical outcomes when delivered as part of a trial than when implemented in routine clinical practice; however, the reasons for this are not fully understood. METHODS/DESIGN: We interviewed 24 individuals who took part in a trial of an innovative health technology under development for people with type 1 diabetes which automatically regulates blood glucose: the closed-loop system. Participants were interviewed following randomisation to a closed-loop and at trial closeout. RESULTS: Participants provided complex agendas for taking part in which altruistic and self-interested considerations were often inseparable. Many described belonging to a wider diabetes community and being beneficiaries of others' participation in research and how this had given rise to attendant citizenship obligations. Participants also shared the excitement and pride they experienced from contributing to research which situated them at the forefront of technological innovation and enabled them to present themselves to others, by virtue of their trial participation, as ambassadors of hope and research pioneers. Given their desire to support the progression of a potentially life-changing technology, and be part of that innovation, participants, at follow-up, described having made extra effort during the trial. Specifically, participants described having been more focused on their diabetes management to help create conditions in which the closed-loop could work most effectively to optimize their blood glucose control. CONCLUSIONS: Our findings contribute a new dimension to understandings of trial effects; specifically, we argue that, to aid interpretation of trial outcomes, participants' understandings and motivations for participation need to be considered. We highlight the potential pertinence of our findings in the contemporary era of bio-citizenship where, increasingly, people are driving research agendas and see themselves as co-producers of knowledge. We also recommend a new concept be introduced into the literature-'the altruselfish agenda'-to recognise potential inseparability of self-interested and altruistic motivations. TRIAL REGISTRATION: ClinicalTrials.gov, NCT02523131 . Registered on 14 August 2015.