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 Asthma Harmonisation project aimed to document all self-reported measures related to asthma in these five British cohort studies and produce harmonised indicators of self-reported asthma through retrospective harmonisation. This harmonisation serves two primary purposes: improving the measurement of asthma (as an outcome, predictor, or control variable) within cohort analyses, and facilitating cross-cohort research on asthma using these rich datasets.Separate longitudinal datasets were created for each cohort, focusing on self-reported data and some measures 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 asthma status of cohort members themselves (not their parents).SN 9420 includes harmonised asthma data for the MRC National Survey of Health and Development only. Harmonised asthma data for NCDS, BCS70, Next Steps and MCS are available under Safeguarded data access from SN 9417. Harmonised indicators of self-reported asthma in the 1946 NSHD and includes sweep-specific indicators and derived cumulative indicators.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.