Background: Immune-checkpoint inhibitors (ICI) are approved for multiple cancers but can result in ICI-associated myocarditis, an infrequent but life-threatening condition. Elevations in cardiac biomarkers, troponin-I (cTnI), troponin-T (cTnT) and creatine-kinase (CK) are used for diagnosis. However, the temporal elevation of these biomarker elevations with course of disease and their association with outcomes have not been established. Methods: We analyzed the diagnostic accuracy and prognostic performances of cTnI, cTnT and CK in ICI-myocarditis (n=61) in two cardio-oncology units (APHP.Sorbonne, France & Heidelberg, Germany). Major adverse cardio-myotoxic events (MACE) were defined as heart failure, ventricular arrhythmia, atrioventricular/sinus block requiring pacemaker, respiratory muscle failure requiring mechanical ventilation, and related death. Diagnostic performances of troponins were also assessed in an international ICI-myocarditis registry (n=244 independent cases, 13 countries). Results: On presentation, cTnT, cTnI or CK were increased compared to upper reference limit (URL) in 51/52 (98%), 28/34 (82%, p=0.009 vs. cTnT), 33/48 (69%, pConclusions. Significant discrepancies between cTnT (compared to cTnI, and CK) circulating levels exist in ICI-myocarditis. cTnT is the best predictor of MACE and most suitable for diagnosis and surveillance. A ratio of cTnT/URL