The large international randomized ASSERT study observed two important results: first, asymptomatic episodes of AF were more frequent in elderly hypertensive patients with pacemakers implanted and no history of clinical atrial fibrillation (AF) and these subclinical atrial tachyarrhythmias were associated with a sudden increase in the risk of ischemic stroke thereafter; and, second, continuous atrial overdrive pacing with programmable algorithms to maintain atrial pacing neither prevented clinical AF and neither reduces stroke risk (N. Engl. J. Med. 2012;366:120-9). The study, conducted by Dr. Jeff S. Healey and colleagues at the Institute for Population Health, McMaster University, Canada, included 2,580 patients from 23 countries. All patients were ≥65 years of age, had hypertension, and had a recently implanted dual-chamber pacemaker or implantable cardioverter-defibrillator. None of the patients had a history of AF at enrollment. At the start of the study, all patients with pacemakers implanted were randomized into 2 groups, one with continuous atrial overdrive pacing for that time period and the other with this function turned off. During the first 3 months of follow-up, device-detected subclinical AF (defined as an atrial rate of more than 190 beats/min for more than 6 min) occurred in 10.1% of patients. Of note, the median time to the onset of this asymptomatic AF in this subgroup of patients was 36 d; therefore, negative Holter ambulatory electrocardiographic monitoring results on consecutive days may have been false-negative. The subgroup of patients who developed asymptomatic AF within the initial 3 months had a 5.6-fold increased risk of developing clinical AF during a mean of 2.5 years of prospective follow-up. In addition, the incidence of ischemic stroke or systemic embolism was 1.7%/year in patients who developed subclinical AF within the initial 3 months, compared with 0.69%/year in the remaining patients in the study. The population-attributable risk (PAR) for asymptomatic AF-related stroke or systemic embolism within the first 3 months was 13%, which is similar to the PAR for clinical AF-related stroke observed in the Framingham Heart Study. A multifactorial analysis correcting for standard predictors of stroke showed that subclinical AF detected by the device within the first 3 months was independently associated with a 2.5-fold increase in the risk of stroke thereafter, which may be an underestimation of the magnitude of the risk because more than half of the patients in the study were treated with aspirin at baseline and 18% of those who developed early subclinical AF were treated with warfarin during the follow-up period, and these 2 medications were clearly effective in reducing stroke in patients with clinical AF. effectively reduce stroke risk in patients with clinical AF, but it is unclear whether they are equally beneficial in patients with subclinical AF. The incidence of ischemic stroke or systemic embolism was 3.7%/year in patients with a CHADS2 score of >2 who had subclinical AF detected in the first 3 months, compared with 0.97%/year in patients with a similarly high CHADS2 score but without early subclinical atrial tachyarrhythmias. In addition to the 10% of patients who developed subclinical AF within the first 3 months of follow-up, an additional 24% developed subclinical AF thereafter.The study did not categorize asymptomatic AF episodes of less than 6 min in duration, and therefore it is not known whether these episodes are also associated with an increased risk of stroke thereafter. In addition, the atrial pacing intervention did not affect the incidence of clinical AF, although the efficacy of the study in testing this prognosis was insufficient.