Arrhythmia and late sudden death are recognized complications of intraatrial repair of transposition of the great arteries. We performed a prospective study over 8 years in 100 consecutive hospital survivors who underwent Mustard (46 patients) or Senning (54 patients) operations between 1978 and 1982. Arrhythmia was analyzed by preoperative, postoperative, and serial follow-up Holter monitoring and standard electrocardiograms and was related to clinical outcome. Before repair, all patients were in sinus rhythm with a low incidence of arrhythmia. After repair, there was a gradual decrease in stable sinus rhythm during follow-up so that at a mean of 7 years after operation only 56% of patients having the Senning operation and 66% having the Mustard, with simple transposition, were in stable sinus rhythm on Holter monitoring. There was no significant difference between the two operations. Eleven patients (five after Senning [two simple, three complex], six after Mustard [five simple, one complex]) died during follow-up, four suddenly (two after Senning, two after Mustard). However, loss of sinus rhythm or the presence of arrhythmia on standard electrocardiograms or Holter recordings did not identify patients at risk for increased morbidity or mortality. Thus, even with current surgical techniques, gradual loss of sinus rhythm occurred after both Mustard and Senning operations. Because late death could not be predicted by electrocardiographic analysis, an alternative approach involving detailed hemodynamic and electrophysiologic measurements may be required to identify high-risk patients.