BACKGROUND Atrial tachyarrhythmias (AT/AF) have been associated with an increased risk of mortality, morbidity and ischemic stroke. Up to now, single chamber ICD diagnostics was not able to detect AT/AF, therefore the incidence of new onset AT/AF in patients with single chamber ICD is not known. OBJECTIVE To evaluate incidence and predictors of AT/AF occurrence in patients with dual-chamber ICD with no pacing indications and no history of AT/AF that strictly mimic single chamber ICD recipient. METHODS & RESULTS Consecutive dual-chamber ICD patients were prospectively followed by 47 Italian cardiologic centers in an observational research. Clinical and device data were reviewed by expert cardiologists to assess AT/AF occurrence. Multivariate regression analysis evaluated the risk of new-onset AT/AF and its association with patients' baseline characteristics and with CHADS2 score. 428 (13.4% female, 64years old) patients were followed for a median observation period of 31months. AT/AF episodes occurred in 160 (37.4%) patients when considering at least 5min duration, in 95 (22.2%) for AT/AF ≥6h, in 47 (11.0%) for AT/AF ≥1day, in 29 (6.8%) for AT/AF ≥7days. Patients with CHADS2≥2, who comprised 36% of the whole population, showed higher incidence of AT/AF ≥6h compared with patients with CHADS2<2 (Hazard Ratio=1.69, 95% Confidence Interval=1.13-2.53, p=0.011). CONCLUSIONS Our observations in a population of dual-chamber ICD patients with no pacing indications and no history of AT/AF, who strictly mimic single-chamber ICD recipients, highlight that AT/AF episodes occurred in the 37.5% of the population and CHADS2 score is predictive of new-onset AT/AF.