IMPORTANCE: Although patients with myocarditis after COVID-19 mRNA vaccination appear to have a good prognosis near hospital discharge, their longer-term prognosis and management remain unknown. OBJECTIVE: To study the cardiovascular complications of post-COVID-19 mRNA vaccination myocarditis and other types of myocarditis during an 18-month follow-up, as well as disease management based on a study of the frequency of medical procedures and drug prescriptions. DESIGN, SETTING, AND PARTICIPANTS: In this cohort study based on the French National Health Data System, all individuals aged 12 to 49 years hospitalized for myocarditis in France between December 27, 2020, and June 30, 2022, were identified. EXPOSURE: Individuals were categorized as having postvaccine myocarditis (within 7 days after COVID-19 mRNA vaccine), post-COVID-19 myocarditis (within 30 days of SARS-CoV-2 infection), or conventional myocarditis. MAIN OUTCOMES AND MEASURES: The occurrence of clinical outcomes (hospital readmission for myopericarditis, other cardiovascular events, all-cause death, and a composite outcome of these events) over the 18 months following hospital admission were analyzed using weighted Cox models to standardize the comparisons with the conventional myocarditis group. Also, medical management after hospital discharge was longitudinally assessed using generalized estimated equation models. RESULTS: In total, 4635 individuals were hospitalized for myocarditis: 558 with postvaccine myocarditis, 298 with post-COVID-19 myocarditis, and 3779 with conventional myocarditis. Patients with postvaccine myocarditis were younger than those with post-COVID-19 and conventional myocarditis (mean [SD] age of 25.9 [8.6], 31.0 [10.9], and 28.3 [9.4] years, respectively) and were more frequently men (84%, 67%, and 79%). Patients with postvaccine myocarditis had a lower standardized incidence of the composite clinical outcome than those with conventional myocarditis (32/558 vs 497/3779 events; weighted hazard ratio, 0.55 [95% CI, 0.36-0.86]), whereas individuals with post-COVID-19 myocarditis had similar results (36/298 events; weighted hazard ratio, 1.04 [95% CI, 0.70-1.52]). The standardized frequency of medical procedures and drugs prescribed in patients with postvaccine myocarditis or post-COVID-19 myocarditis followed a similar trend in the 18 months following hospital discharge to that of patients with conventional myocarditis. CONCLUSIONS AND RELEVANCE: Patients with post-COVID-19 mRNA vaccination myocarditis, contrary to those with post-COVID-19 myocarditis, show a lower frequency of cardiovascular complications than those with conventional myocarditis at 18 months. However, affected patients, mainly healthy young men, may require medical management up to several months after hospital discharge.