Before Mustard's operation 27 children, from 2 days to 11 months old, with simple or complex transposition of the great vessels, underwent an electrocardiogram at rest. After operation, all the children underwent a standard electrocardiogram and 24 hours continuous Holter monitoring. Successive electrocardiograms at rest were recorded in all patients, and eleven children were successfully examined by dynamic electrocardiogram. The electrocardiogram at rest showed a sinusal rhythm in 66.7 of the patients; during the follow-up, in 33.3% non sinusal rhythm was recorded. The continuous 24 hours dynamic electrocardiogram, immediately after surgery, revealed 25.9% of the patients with non-sinusal rhythm. After 7 months, the Holter monitoring showed, in one case, a severe sinus node disfunction. Our data and those of the literature confirm the importance to follow these patients during a long period of time, because it was found that, even bedately, can develop serious arrhythmias.