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Background Ethnic inequalities exist in the management of patients with cancer with acute coronary syndrome (ACS). Given their under-representation in trials, ethnic minority patients are often studied using large registries, but the quality of ethnicity coding in these datasets remains unclear.Methods Agreement of ethnicity coding and outcomes for patients with cancer with ACS (2000–2018) was examined across four national datasets: National Cancer Registration and Analysis Service (NCRAS), Myocardial Ischaemia National Audit Project (MINAP), British Cardiovascular Intervention Society database (BCIS) and Hospital Episode Statistics (HES). Three linkages were performed: NCRAS-MINAP, NCRAS-MINAP-BCIS, NCRAS-MINAP-HES, with four groups based on ethnicity agreement: Concordant, Discordant, Missing (1 and ≥2 datasets). Multivariable logistic regression and Cox’s Proportional Hazards models assessed 1-year and long-term (≤5 years) cardiac and cancer-related death for each agreement group.Results Among three linkages, just over half of the ethnicities were concordant (range: 52.4%–53.8%). Discordance was relatively low (range 1.2%–5.5%) while missingness ranged between 28.6% and 43.4% in 1 dataset and 1.6%–12.6% in ≥2 datasets. Ethnicity correlation between individual datasets was poor, lowest between NCRAS and BCIS (r=0.318). We observed higher 1-year and long-term cardiac and cancer deaths in several of the Missing (1 and ≥2 datasets) groups across the three linkages, compared with the Concordant group.Conclusion Across four national datasets for patients with cancer with ACS, nearly half of patients had missing ethnicity in at least one dataset, which was associated with higher cardiac or cancer mortality. Inconsistency in ethnicity coding represents a missed opportunity to examine health inequalities in this high-risk and understudied population.