AIMS: Concerns have been raised regarding the potential causal relationship between myocarditis and mRNA COVID-19 vaccines. In response, the Korea Disease Control and Prevention Agency actively monitored myocarditis and pericarditis in adolescents who received mRNA COVID-19 vaccines in South Korea. Understanding the long-term health outcomes of these cases is essential for developing evidence-based national health policies. We aimed to identify the health-related quality of life (HRQoL) through long-term follow-up observations of myocarditis and pericarditis in this population. METHODS AND RESULTS: This cross-sectional descriptive study was conducted utilizing a self-reported online structured survey. Adolescents in South Korea diagnosed with myocarditis or pericarditis after receiving mRNA COVID-19 vaccination were recruited from those who received compensation decisions from the Korea Disease Control and Prevention Agency. Health-related quality of life, depression, anxiety, subjective health status, and health self-efficacy were measured, and the correlations and differences among these variables were analysed. The 45 participants obtained an average self-reported HRQoL score of 81.6 out of 100 points. The mean depression score was 13, with 44.4% of the participants reporting an increase in depressive symptoms, following their diagnosis of myocarditis or pericarditis. Anxiety, with an average score of 2.96, was reported to have increased by 51.1% after the diagnosis. The mean health self-efficacy score was 59. CONCLUSION: These results suggest that individuals experiencing myocarditis or pericarditis following COVID-19 vaccination may encounter physical and psychological challenges, underscoring the need for comprehensive support and evaluation.