Abstract Aims Guidelines do not differentiate between the available assays of cardiac troponin (cTn). We compared the prognostic and predictive ability of cTn assays. Methods and results This was a nationwide cohort study of patients with acute coronary syndrome (ACS) and ≥ 2 cTn measurements of one of four assays: Roche high-sensitivity cTnT (hs-cTnT), Abbott high sensitivity cTnI (hs-cTnI), Siemens Vista cTnI, and Siemens cTnI Ultra. Data were collected from Danish registries from 2009–18. Peak cTn concentration normalized to the 99th percentile was used. Outcomes were myocardial infarction (MI) during admission, one-year all-cause-, cardiovascular-, and non-cardiovascular mortality. Receiver operating characteristics and logistic regression calculating odds ratios (OR) were used. A total of 90 705 patients were included, of which 20 550 (23%) had MI. Siemens Vista cTnI was the strongest predictor of MI, Area under the curve (auc) 0.93 (95% CI 0.93–0.93). In 1 year 9012 (9.9%) of patients had died. An inverted U-shape relationship was observed between concentration of cTn and all-cause mortality. Hs-cTnT OR 21.3 (95% CI 18.4–24.8) at 2–5 times the 99th percentile and 12.1 (95% CI 10.3–14.1) for concentrations >100 times the 99th percentile. The inverted U-shape relationship was only present for non-cardiovascular mortality. The strongest predictor of cardiovascular mortality was hs-cTnT, OR 11.3 (95% CI 6.4–21.8) at 1–2 times the 99th percentile and 88.8 (95% CI 53.2–163.0) for concentrations >100 times the 99th percentile. Conclusion Siemens Vista cTnI was the strongest predictor of MI and hs-cTnT was the strongest predictor of mortality. An inverted U-shape relationship was observed between cTn concentration and non-cardiovascular mortality.