BackgroundData-sharing is increasingly encouraged or required by funders and journals. Data-sharing is more complicated for lifecourse studies that rely upon ongoing participation, but little is known about perspectives on data-sharing among participants of such studies. The aim of this qualitative study was to explore perspectives on data-sharing of participants in a birth cohort study.MethodsSemi-structured interviews were conducted with 25 members of the Dunedin Multidisciplinary Health and Development Study when aged between 45 and 48 years. Interviews were led by the Director of the Dunedin Study and involved questions about different scenarios for data-sharing. The sample consisted of nine Dunedin Study members who are Māori (the Indigenous peoples of Aotearoa/New Zealand) and 16 who are non-Māori.ResultsPrinciples of grounded theory were applied to develop a model of participant perspectives on data-sharing. The model consists of three factors that inform a core premise that a one-size-fits-all approach to data-sharing will not suffice in lifecourse research. Participants suggested that data-sharing decisions should depend on the cohort and might need to be declined if any one Dunedin Study member was opposed (factor 1). Participants also expressed a proven sense of trust in the researchers and raised concerns about loss of control once data have been shared (factor 2). Participants described a sense of balancing opportunities for public good against inappropriate uses of data, highlighting variability in perceived sensitivity of data, and thus a need to take this into account if sharing data (factor 3).ConclusionsCommunal considerations within cohorts, loss of control over shared data, and concerns about inappropriate uses of shared data need to be addressed through detailed informed consent before data-sharing occurs for lifecourse studies, particularly where this has not been established from the start of the study. Data-sharing may have implications for the retention of participants in these studies and thus may impact on the value of long-term sources of knowledge about health and development. Researchers, ethics committees, journal editors, research funders, and government policymakers need to consider participants’ views when balancing the proposed benefits of data-sharing against the potential risks and concerns of participants in lifecourse research.