The modified Fontan procedure for univentricular heart disease results in the full spectrum of significant cardiac arrhythmias: tachycardias and bradycardias. The tachycardias are primarily supraventricular in origin: 1. atrial flutter, 2. primary atrial tachycardia, 3. atrial fibrillation, and 4. accelerated junctional rhythm or junctional tachycardia; however, ventricular tachyarrhythmias occur also, but less frequently. Bradycardia usually is a result of 1. sinoatrial node dysfunction resulting in junctional rhythm or 2. less commonly, atrioventricular conduction abnormalities, such as second degree and third degree atrioventricular block. Finally, the combination of atrial tachyarrhythmias and sinoatrial node dysfunction can occur in a small but significant number of patients after Fontan surgery. A complete array of medical and pacemaker therapeutic options is necessary for the physician to treat these difficult arrhythmias successfully.