Health examinations are an essential part of cohort studies: questionnaires are filled in, biological samples drawn, bodies weighed and measured, their capacities and functions tested. Drawing on an ethnography of these clinical encounters, in the context of a population-based environmental health cohort in Switzerland, I describe the choreography of data production and how it blurs the boundary between healthcare and scientific research. In contrast to the notion of clinical labour, which describes logics of objectification and extraction, this fieldnote paints a more nuanced and sensitive picture, in which care work performed by nurses, the active role played by cohortees, and the materialities around them, come together and move apart. These fragile choreographies point to the importance of care work as a form of expertise necessary for data production.