Ojectives. This study was undertaken to determine the potential role of P wave signal-averaged electrocardiogram (PSAECG) for risk assessment of atrial tachyarrhythmias (ATs) in patients after Fontan operation. Background. Onset of atrial flutter/fibrillation (AFF) in patients who have undergone Fontan operation for univentricular hearts constitutes an unfavorable clinical event associated with a high risk of cardiovascular complications. There is no data available on PSAECG in postoperative Fontan patients to predict potential susceptibility to ATs. Methods. Twenty-four post-Fontan patients and 15 age-matched healthy subjects were prospectively studied with PSAECG, and the following measurements were made: filtered P wave duration (FPWD), P wave vector integrals (PINTs), root-mean-square voltage for the initial 30 ms (RMSi30), and duration of persistent amplitude signals <4 μV from the beginning of the P wave (Di4). Results. The FPWDs were significantly longer in the study group patients with ATs when compared with the study group patients without ATs (p < 0.01) and compared with the controls (p < 0.001). An FPWD cut point of 135 ms resulted in a sensitivity of 71% and a specificity of 81% in differentiating patients with ATs from patients without ATs among the postoperative Fontan patients. The PINT was significantly greater in Fontan patients with AFF and also without AFF when compared with controls (p < 0.01, p < 0.05, respectively). The RMSi30 and the Di4 were not significantly different between study and control groups. Conclusions. Signal-averaged P wave duration is significantly prolonged in postoperative Fontan patients. A prolonged signal-averaged P wave duration may be an effective noninvasive marker to predict risk of development of ATs in this patient group. (C) 2000 by the American College of Cardiology.