The Mustard and Senning procedures may be complicated by atrial tachyarrhythmias. In order to determine their prognosis, the authors undertook a review of the 288 patients operated and followed up in their department. Thirty-six (12.5%) had documented atrial tachycardias. In 13 cases, Holter monitoring showed bursts of atrial tachycardia. In the other 23 cases, the tachycardia was sustained and often caused severe symptoms (syncope). Reduction of the sustained forms was obtained by rapid atrial pacing, cardioversion or amiodarone therapy. After restoration of sinus rhythm, several therapeutic protocols were used, often in the same patient: abstention, leading to 5 recurrences in 6 cases; treatment with betablockers in 12 patients with well tolerated or exercise-induced atrial tachycardia with 11 successes; amiodarone, with 4 relapses out of 5 when the dosage was less than 200 mg/m2/day and 13 successes out of 18 when the dosage was 200-250 mg/m2/day. During a follow-up of 1 to 19 years, 6 severe complications (neurological sequellae, cardiac failure), and 6 deaths, were observed. All these patients were poorly controlled and continued to be have tachyarrhythmias. The authors conclude that tachyarrhythmias are neither rare nor always symptomatic, so justifying their systematic investigation. They should be reduced by oral amiodarone or rapid atrial pacing depending on the clinical urgency, never by intravenous antiarrhythmics which can be dangerous. The poor prognosis associated with tachyarrhythmias makes their control essential either with betablockers or good doses of amiodarone, eventually covered by permanent cardiac pacing.