The cardiac rhythm before and after the modified Fontan procedure was reviewed in 24 patients. Transient atrial dysrhythmias were common in the immediate postoperative period. Late postoperative premature atrial contractions were detected by ambulatory monitoring in 20 of 23 patients; eight (34.8%) had supraventricular tachycardia. Late ventricular dysrhythmia was detected in 18 of 23 patients: ten had low-grade ventricular dysrhythmias and eight (34.8%) had multiform premature ventricular contractions. Five of the latter had couplets and one of these five plus another had ventricular tachycardia. Seven patients with supraventricular tachycardia and five patients with ventricular dysrhythmia required antiarrhythmic medication. Asymptomatic bradycardia was detected in five patients (21.7%). One patient had intermittent second-degree atrioventricular block. No specific risk factors predicted dysrhythmias. Thus, cardiac dysrhythmias were common in patients after the modified Fontan procedure, but were well tolerated in most patients. No sudden deaths or syncopal episodes have occurred during a mean follow-up of 5 years. One patient's death was related to severe left ventricular dysfunction. Permanent pacing has not been required in any patient.