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Introduction: Cardiac magnetic resonance (CMR) has excellent diagnostic accuracy in patients with suspected myocarditis. The role of CMR-assessed strain (using feature tracking; FT) in myocarditis assessment is evolving. Methods: We included consecutive patients with clinical suspicion of acute myocarditis referred for CMR between 2012 and 2017. We compared patients with confirmed diagnosis of acute myocarditis (n=59, age 40±15, 59% male), patients in whom myocarditis was excluded (n=28, age 40±13, 55% male), and age- and gender-matched controls with normal CMR (n=59). We quantified myocardial mechanics using FT-strains and T1 mapping using dedicated software. Investigators were blinded to clinical data. The primary endpoint was MACE: death, cardiac rehospitalization, or implantation of ICD-CRT-LVAD. Results: Patients with myocarditis exhibited more impaired longitudinal strain (FT-LS) than those without (14.8±5.6 vs 21.9±2.9 ms, p<0.01) or controls (14.8±5.6 vs 23.4±2.7 ms, p<0.01). FT-LS was not different between the latter two groups. Myocarditis patients in the worse tertile of LS showed more severity of myocarditis than those in the best tertile (Table): longer hospitalization, more use of diuretic and inotropic drugs, more severe cardiac damage (as reflected by higher peak TnI and BNP, and shorter post-contrast T1 time), worse baseline systolic and diastolic function, and worse recovery of LVEF at follow up. Over a mean follow up of 30±15 months, 23 patients reached the primary endpoint. FT-LS >-11.5% was predictive of the primary endpoint (sensitivity 78%, specificity 90%, area under the curve 0.82). In multivariate analysis, LS was an independent predictor of MACE even after adjusting for initial LVEF and TnI (HR 3.38, [2.05-4.84], p<0.01). Conclusions: CMR-derived LS using FT is an independent predictor of adverse cardiovascular outcome, and is thus an effective tool to evaluate the severity and prognosis of patients with acute myocarditis. [ABSTRACT FROM AUTHOR]