["Sociology of Health & Illness, EarlyView. ", "\nAbstract\nIn this article, we address the problem of participation and the dominant focus on motivations in clinical research. We explore participation as a relational mode of ‘being in time’ in Alzheimer's dementia prevention—a field profoundly shaped by changing bodies through time, as well as promissory trends towards future‐oriented preventative medicine. Analysis of interviews with older adults in a clinical trial platform demonstrates that what research ‘does’ or might (not) ‘do’ for participants emerges as temporalities of participants’ everyday lives become entangled with the possibilities, constraints and demands of biomedical ‘research time’. As well as consistent desires to help (future) others, we identify incidental possibilities for care that emerged from continued research participation. We argue that longitudinal research participation can productively be understood as a set of evolving affordances: whereby differing limits and possibilities for care and agency emerge in a world where dementia cannot be cured. Future trial participation is considered in terms of ‘therapeutic affordances’, which are likely to fluctuate as certain lived or imagined futures unfold. As such, we open up a conceptual space to think about why, how, and critically, when participation happens, as it emerges in relation to lived times of ageing and everyday life.\n"]