Atrial fibrillation is a very common arrhythmia in patients who need an implantable cardioverter-defibrillator (ICD) because of life threatening ventricular tachyarrhythmias. The prevalence of atrial fibrillation at the time of implantation has been calculated to be as high as 20% and it has been reported that during the lifespan of the ICD more than 50% of patients may develop atrial fibrillation.1 Atrial fibrillation may lead to inappropriate ventricular shocks,2 ventricular arrhythmia induction,3 and thromboembolism after ventricular shocks in the presence of unknown atrial fibrillation. Furthermore, data from the AVID (antiarrhythmic versus implantable defibrillators) registry showed that atrial fibrillation was an independent predictor of worse survival.4 In our series (151 patients), 20% of patients had atrial fibrillation before implantation and 31% suffered from the first episode during a mean (SD) follow up of 24 (19) months. In fig 1, survival free of atrial fibrillation after ICD implantation in our series may be appreciated.