Objective Incidence and severity of acute myocarditis vary significantly in previous reports and there is a lack of epidemiological studies on the short-term risks of mortality, heart failure and ventricular arrhythmias in patients with acute myocarditis. Therefore, study aims were to examine 90-day risks of mortality, heart failure (HF) and ventricular arrhythmias in patients with acute myocarditis in comparison to age-matched and sex-matched background population controls. Methods In this nationwide register-based follow-up study of patients hospitalised with myocarditis between 2002 and 2018 in Denmark, 90-day risks of all-cause mortality, HF, ventricular arrhythmias (ventricular tachycardia, ventricular fibrillation (VF)), cardiac arrest and implantable cardioverter-defibrillator (ICD) implantation were compared with age-matched and sex-matched controls from the background population (1:5 matching). Absolute risks standardised to the age, sex and comorbidity distribution of the entire study population were derived from multivariable Cox regression. Results A total of 2523 patients hospitalised with myocarditis were included. Median age was 48 years (Q1-Q3: 30-69) and 67.7% were men. Comorbidity burden was more pronounced among patients with myocarditis relative to controls. Standardised 90-day all-cause mortality risk was 4.9% for patients with acute myocarditis versus 0.3% for controls (p<0.001). Ninety-day standardised risks for other endpoints were 7.5% versus 0.1% for HF, 1.9% versus <0.1% for VF/VF/arrest risk and 1.6% versus <0.1% for ICD implantation (all p<0.001). Conclusions In this large nationwide register-based follow-up study, patients hospitalised with myocarditis had significantly higher 90-day risks of all-cause mortality, HF, ventricular arrhythmias, cardiac arrest and ICD implantation compared with background population controls.