The long-term natural progression of cardiac rhythm and the incidence of serious arrhythmias in relation to previous procedures and associated heart defects were studied in a group of 76 patients after an arterial switch operation for complete transposition and compared to a group of 24 patients who had undergone intraatrial corrections (Mustard or Senning operation). Standard and 24-hour ambulatory electrocardiograms were reviewed. Mean follow-up was 7.9 years (range 2-16) after an arterial switch and 15.2 years (range 6-26) following the Mustard or Senning procedures. One patient died after an arterial switch from pulmonary hypertension (age 9.9 years), and three patients died suddenly, presumably from arrhythmias, following atrial redirection procedures (ages 13, 18 and 20 years). Symptomatic arrhythmias were not seen after the arterial switch operation. Three patients required pacemaker implantation after atrial redirection, and a further two required medication to control tachyarrhythmias. Survival analysis of sequential electrocardiograms showed a mean maintenance of sinus rhythm during 12.9 years (95% confidence interval 11.4-14.5) after the arterial switch and 9.0 years (7.3-10.7) after atrial procedures (p=0.003). Development of heart rate was significantly different (p=0.001), with higher rates in adolescents after an arterial switch. Twenty-four-hour recordings were abnormal in five of 72 patients following arterial correction, disclosing excessive ventricular extrasystoles in four (three monomorphic, one polymorphic) and a wandering pacemaker in one. After atrial procedures, 11 of 19 were abnormal (p<0.001), with sinus or atrial bradycardia in eight, atrial flutter in two, and monomorphic ventricular tachycardias in one. Abnormal findings on either 12-lead or 24-hour electrocardiograms were seen in 22% of patients following an arterial switch procedure and 83% of patients undergoing atrial redirection (p<0.001). Sinus rhythm, therefore, is preserved longer and arrhythmias are less frequent and less severe after the arterial switch operation than after the Mustard or Senning operations.