The Harmonised Data in Five National Longitudinal Cohort Studies project, supported by the Economic and Social Research Council (ESRC), brings together data from five British cohort studies: the 1946 National Survey of Health and Development (NSHD), the 1958 National Child Development Survey (NCDS), the 1970 British Cohort Study (BCS70), Next Steps (formerly the Longitudinal Study of Young People in England), and the Millennium Cohort Study (MCS). NCDS, BCS70, Next Steps, and MCS receive core funding from the ESRC and are hosted by the Centre for Longitudinal Studies at UCL. NSHD is funded by the Medical Research Council (MRC) and is hosted by the Unit for Lifelong Health and Ageing at UCL. The Diabetes Harmonisation project aimed to document all self-reported measures related to diabetes in these five British cohort studies and produce harmonised indicators of self-reported diabetes through retrospective harmonisation. This harmonisation serves two primary purposes: improving the measurement of diabetes (as an outcome, predictor, or control variable) within cohort analyses, and facilitating cross-cohort research on diabetes using these rich datasets.Separate longitudinal datasets were created for each cohort, focusing on self-reported data with some information derived from doctors' reports and medical records. Biomarkers and linked health data were not included. Focus was given to measures that were administered to entire cohorts only and on the diabetes status of cohort members themselves (not their parents or children).SN 9421 includes harmonised diabetes data for the MRC National Survey of Health and Development only. Harmonised diabetes data for NCDS, BCS70, Next Steps and MCS are available under Safeguarded data access from SN 9416. Harmonised indicators of self-reported diabetes in the 1946 NSHD and includes sweep-specific indicators, derived cumulative indicators, and derived diabetes type.The research IDs differ from the original research IDs and data can therefore not be merged with other NSHD data files. Access to the harmonised variables in connection with other NSHD variables can be requested via the usual NSHD data sharing request.