Background The success of health research depends on the involvement of participants. Few studies have examined the participation of subjects in cohorts. Drawing on a sociological survey on a French cohort around aging, this study proposes to question the nature of interactions between researchers and subjects that would help to understand the motivations of subjects to participate and remain in health research studies. Methods Qualitative study combining participant observation within the research laboratory that conducted the cohort and semi-structured interviews with subjects included in the cohort and with members of the research team. Results This study highlights the existence of two-way care: from the laboratory to the subjects and from the cohort to researchers. Health research seems to correspond to a complex association between subjects concerned with aging and the expected benefits of exceptional monitoring. Research is incorporated into subjects’ daily lives, allowing a shift in the purpose of research from overmedicalization to medical safety that subjects experienced as a form of care. Conclusions In cohort studies, care is understood as a form of attention to the person through high-quality medical follow-up. Aging is turned into a matter of concern that subjects, in collaboration with researchers, strive to control.