Two cases of arrhythmogenic right ventricular dysplasia (ARVD) in siblings are reported. In the boy, 14 years old, the clinical history, ECG, echocardiography, and histopathological findings were consistent with ARVD. Premature ventricular contractions of left bundle branch block (LBBB) pattern were recorded but no ventricular tachycardia (VT). A high titer against mycoplasma and increased concentrations of immunoglobulins were found. Two years after his first admission he died suddenly. Autopsy revealed severe right ventricular (RV) myocardial damage, with fat cell infiltration and collagenous tissue. His sister presented with sustained VT of LBBB pattern 2 years later, at 12 years of age. Vaccination against rubella and signs of upper respiratory illness had preceded the symptoms. During the following 9 days ECGs and serum enzymes indicated the development of left ventricular (LV) infarction. Echocardiography revealed an enlarged RV and a normal LV. After 6 weeks both RV and LV showed akinetic areas and sacculations. We suggest that myocarditis may be a precipitating factor in ARVD, and perhaps the prerequisite for its manifestation.