Atrial fibrillation (AF) and obesity are coinciding epidemics. Obstructive sleep apnea (OSA) correlates directly with obesity and is highly prevalent among middle-aged adults. Emerging evidence supports a strong association between AF and OSA. The rate of AF among patients with OSA is low but is nevertheless higher than that in the general population. The prevalence of OSA among patients with AF is strikingly high-at least 32% and possibly as high as 49%-although differences in the AF populations studied and the criteria used to diagnose OSA have prevented more accurate measurement of this rate. Data in focused populations at risk of AF show that OSA increases the frequency and recurrence of AF. These findings cannot, however, be reliably generalized to the larger OSA population. Although the available information is limited, treatment of OSA, particularly with continuous positive airway pressure, seems to substantially reduce the frequency of AF. If true, this approach would provide a safe and noninvasive therapy for some patients with AF. Randomized controlled trials are necessary to rigorously answer this question. Many data have been obtained from studies with focuses other than the direct relationship between OSA and AF. Studies carefully designed to investigate the relevant issues are now needed.