A 15.5-year-old boy, an athlete, reported to his general practitioner due to episodes of chest pain during exercise, weakness and a feeling of irregular heartbeat that had been occurring for a month. He reported episodes of fainting in the past. Physical examination revealed arrhythmia and the patient was referred for a cardiological consultation, and then admitted to a reference cardiology department for extended cardiac diagnosis. Approximately 66,000/day (49.8%) of premature ventricular beats were recorded in 24-hour Holter electrocardiogram, including episodes of non-sustained ventricular tachycardia. Echocardiography revealed left ventricular systolic dysfunction with a decrease in its ejection fraction to 51.5%. Magnetic resonance imaging of the heart was performed, which showed foci of late post-contrast enhancement, located subepicardialy in the side wall of the left ventricle, corresponding to post-inflammatory changes in the myocardium. Left and right ventricular ejection fraction on magnetic resonance imaging were 48% and 46%, respectively. Pharmacological treatment included a beta-blocker (metoprolol), propafenone and an angiotensin converting enzyme inhibitor (enalapril). During the treatment, ventricular arrhythmia subsided almost completely, a gradual improvement in left ventricular systolic function was observed, and the patient was relieved of pain. The boy was discharged home with a recommendation to continue pharmacological treatment, lead a sparing lifestyle, and postpone practicing sports. At present, the boy’s general condition is good, he does not report any complaints, takes medications regularly, and does sports only for recreation. A follow-up Holter showed only single premature ventricular beats, while echocardiography showed an improved left ventricular systolic function (ejection fraction 63%), which is a good prognostic indicator.