A case of QT interval prolongation with ventricular tachycardia and torsade de pointes is reported. Arrhythmias occurred in a baby with persistent 2:1 atrioventricular block and long QT interval 2 days after birth and were self-limiting. No structural cardiac defect was present. Serum levels of sodium, potassium, magnesium and calcium were in the normal range. Finally, the pathogenetic mechanism of cardiac block is discussed.