Introduction. New arrhythmias (NA) may appear late after accessory pathway (AP) ablation, but their relation to curative radiofrequency (RF) lesion is unknown. Objective. The aim of this study was to determine the prevalence and predictors for NA occurrence after AP ablation and to investigate pro-arrhythmic effect of RF. Methods. Total of 124 patients (88 males, mean age 43±14 years) with Wolff-Parkinson-White syndrome and single AP have been followed after successful RF ablation. Post-ablation finding of arrhythmia, not recorded before the procedure, was considered a NA. The origin of NA was assessed by analysis of P-wave and/or QRS-complex morphology, and, thereafter, it was compared with locations of previously ablated APs. Results. Over the follow-up of 4.3±3.9 years, NA was registered in 20 patients (16%). The prevalence of specific NAs was as follows: atrioventricular (AV) block 0.8%, atrial premature beats 1.6%, atrial fibrillation 5.4%, atrial flutter 0.8%, sinus tachycardia 4.8%, ventricular premature beats (VPBs) 7.3%. Multivariate Cox-regression analysis identified (1) pre-ablation history of pathway-mediated tachyarrhythmias >10 years (HR=3.54, p=0.016) and (2) septal AP location (HR=4.25, p=0.003), as the independent predictors for NA occurrence. In four NA cases (two cases of septal VPBs, one of typical AFL and one of AV-block) presumed NA origin was identified in the vicinity of previous ablation target. Conclusion. NAs were found in 16% of patients after AP elimination. In few of these cases, late on-site arrhythmic effect of initially curative RF lesion might be possible. While earlier intervention could prevent NA occurrence, closer follow-up is advised after ablation of septal AP.