In recent years, Pulmonary Vein Isolation (PVI) has progressively increased its rolein the treatment of drug resistant Atrial Fibrillation (AF). Traditionally these procedures are performed with a “point by point” ablation technique by means of radiofrequency (RF) energy; however, in the last years, the use of novel alternative technologies such as cryothermal balloon (CB) ablation is growing rapidly. Irrespective of the technique used, the first step in every PVI ablation is the access to the left atrium (LA) through a transseptal puncture (TS). In the RF ablation procedure, LA access is commonly achieved with a double TS puncture to insert both an ablation and a circular mappingcatheter. In the CB ablation procedure, only a single TS puncture is usually required.Objective. Our aim was to compare the incidence of iatrogenic atrial septal defect (iASD) between double transseptal conventional RF and CB ablation at 1-year follow-up evaluating its clinical significance We analysed and compared the presence of iASD and its clinical repercussion in aretrospective study of 127 patients, affected by drug resistant paroxysmal or early persistent AF, who underwent PVI by the means of either RF or CB ablation, between January 2008 and December 2012 in UZ Brussels hospital. Transoesophageal echocardiography was performed before each procedure. This study has been followed up for 1 yearResults. The incidence of iASD at 1-year follow up following PVI was significantly higher in the CB group (22.2% vs 8.5%; p=0.03). Mean iASD diameter was larger in the CB group (0.60x0.50cm vs 0.44x0.35cm) without statistical significance. Only left to right atrial shunt was observed. No adverse events were recorded in these patients during the follow upConclusion. The incidence of iASD at 1-year follow up following CB ablation procedure for PVI is significantly higher with respect to RF procedures. Although no adverse clinical events were recorded in patients with persistence of iASD, this complication should not be underestimated and systematic echocardiographic examinations might be advised in all individuals exhibiting this finding