Late postoperative arrhythmias are a common problem associated with significant morbidity following Fontan operations. We analyzed in our patients whether age at the time of a modified Fontan procedure (total cavopulmonary connection) has an influence on the frequency of late postoperative arrhythmias. We examined all 66 long-term surviving patients who underwent a total cavopulmonary connection (TCPC) in our center during a nine year period. At the time of TCPC, 31 children were younger (group I) and 35 children were older than 4 years (group II). The follow-up period was almost identical in both groups (group I: 4.29 years, group II: 4.52 years). We evaluated all ecgs performed at maximum intervals of 6 months and all Holter ecgs performed at intervals of 12 months. Atrial tachyarrhythmias (supraventricular tachycardias, atrial flutter) were registered in six patients (9%). All of these patients belonged to group II (17%). Ventricular arrhythmias (couplets, ventricular tachycardia) were found in 3% of the younger and 17% of the older patients. Abnormalities of sinus node function were registered in equal frequency in younger and older children (group I: 35%, group II: 31%), while pauses with a duration of > 2 s where present only in older children (group II: 14%). Pacemaker implantation or antiarrhythmic therapy became necessary in 10/66 patients (15%) affecting none of the children who underwent the TCPC < 4 years (group I) but 10/35 children (group II) who underwent the TCPC at an older age (29%). According to our findings in the medium-term follow-up after TCPC, atrial tachyarrhythmias and complex ventricular arrhythmias are registered less frequently in children who underwent the modified Fontan procedure at an age of less than 4 years. Although further studies will have to show whether these differences persist in the long-term follow-up, these findings represent an additional argument for the elective performance of modified Fontan operations at an age of less than 4 years.