Premature ventricular contractions are common, both in the general population and among athletes. These arrhythmias are characterized by a heterogeneous aetiology and different clinical significance. They may develop not only as a sign of structural heart disease (e.g. myocarditis, cardiomyopathy, myocardial ischaemia, heart defects), genetically determined ion channel dysfunctions, but also as a result of electrolyte imbalances and adverse effects of pharmacotherapy. However, it is not possible to determine their causes in some of the patients. Premature ventricular contractions in patients with heart disease may be a trigger for life-threatening arrhythmias, especially during physical exercise. Antiarrhythmic agents having various mechanisms of  action, particularly β-adrenergic blocking agents and, in some cases, invasive procedures such as ablation of  an arrhythmogenic focus, are used for the treatment of ventricular arrhythmias. Some of the dietary supplements, polyunsaturated omega-3 fatty acids in particular, may also be effective. Epidemiological and experimental studies as well as randomized clinical trials indicate that n-3 polyunsaturated fatty acids show antiarrhythmic properties by direct effects on electrophysical processes in cardiomyocytes – both atrial and ventricular. Omega-3 polyunsaturated fatty acids were observed to have, among other things, the following effects: inhibiting some of the ionic currents (sodium, L-type calcium, potassium), increasing parasympathetic tone and decreasing the sensitivity of β-adrenergic receptors. This paper presents a case of idiopathic ventricular arrhythmia in a 15-year-old female swimmer successfully treated with n-3 polyunsaturated fatty acids.