The purpose of this study was to establish a relationship between the atrial activation assessed by signal-averaged P wave ECG (SAPW) and episodes of paroxysmal atrial fibrillation (PAF) in patients (pts) with WPW syndrome (WPWS). fourteen pts with documented episodes of PAF, without underlying heart disease were selected (12 men: age = 35.2 ± 11.1 years) from a total of 70 consecutives pts successfully submitted to radiofrequency catheter ablation of accessory pathway (AP). All pts with anterograde conduction over the AP. left side AP in 9 pts and right side AP in 5 pts. The SAPW were recorded with the Corazonix Predictor II system from the X, Y, Z leads (40–250 Hz, least Square filter, final noise < 0.3 μV). The total filter P wave duration (PWD) was obtained manually from the vector-magnitude in ms. Recordings were performed between 7 th and 14 th day after the ablation. No pts showed left atrial larger than 4.0 cm on the echocardiogram. The results were compared with a control group of 12 pts matched in age (5 men; age = 33.0 ± 10.2 years) without heart disease. Values were given as mean and 1SO and compared using non-paired Student t test (significance < 0.05). Control Group In = 12): PWD ranged from 115.0 to 129.5 ms with a mean of 124.4 ± 4.4 ms. WPWS with PAF (n = 14): PWD ranged from 130.0 to 171.0 ms with a mean of 142.9 ± 1 0.1 ms (p < 0.000). Eleven pts (78.5%) showed PWD > 135.0 ms. There were no differences between PWD and AP location. Recurrent PAF after ablation was noted in 3 pts (21% – PWD: 171.0, 1505 and 139.0 ms for each patient). With a cutoff < 135.0 ms for a normal PWD, the sensitivity was 78.5%, the specificity was 100%; the positive predictive value was 100% and the negative was 75% for SAPW to identify pts with WPWS and PAF. In the current study, pts with WPWS and PAF showed prolonged intra-atrial conduction time when compared with a control group. This may contribute to the development of PAF.