ObjectivesTo assess the degree to which we can replicate a study between a regional and a national database of electronic health record data in the UK. The original study examined the risk factors associated with hospitalisation following COVID-19 infection in people with diabetes.DesignA replication of a retrospective cohort study.SettingObservational electronic health record data from primary and secondary care sources in the UK. The original study used data from a large, urbanised region (Greater Manchester Care Record, Greater Manchester, UK-2.8 m patients). This replication study used a national database covering the whole of England, UK (NHS England's Secure Data Environment service for England, accessed via the BHF Data Science Centre's CVD-COVID-UK/COVID-IMPACT Consortium-54 m patients).ParticipantsIndividuals with a diagnosis of type 1 diabetes or type 2 diabetes prior to a positive COVID-19 test result. The matched controls (3:1) were individuals who had a positive COVID-19 test result, but who did not have a diagnosis of diabetes on the date of their positive COVID-19 test result. Matching was done on age at COVID-19 diagnosis, sex and approximate date of COVID-19 test.Primary and secondary outcome measuresHospitalisation within 28 days of a positive COVID-19 test.ResultsWe found that many of the effect sizes did not show a statistically significant difference, but that some did. Where effect sizes were statistically significant in the regional study, then they remained significant in the national study and the effect size was the same direction and of similar magnitude.ConclusionsThere is some evidence that the findings from studies in smaller regional datasets can be extrapolated to a larger, national setting. However, there were some differences, and therefore replication studies remain an essential part of healthcare research.