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Background: Obstructive sleep apnea (OSA) is a common disorder that affects 5% of the adult North American population. It is associated with atrial arrhythmias and stroke. The mechanisms of this association remain unclear. The aim to the study was to identify the factors associated with interatrial block (IAB) among patients with OSA., Methods: Patients referred for polysomnography were studied. Sleep apnea severity (apnea-hypopnea index [AHI]) was measured in each subject. 12-lead ECGs were scanned and amplified (× 10); P-wave duration and dispersion were measured using a semi-automatic caliper. IAB was defined as a P-wave duration ≥ 120 ms., Results: Data from 180 consecutive patients was examined. Moderate-severe OSA (mean AHI = 56.2 ± 27.9) was present in 144 (OSA group). The remaining 36 had mild or no OSA (mean AHI = 5.6 ± 3.6) and were used as controls. Age distribution between the groups did not differ and there were more males in the OSA group (69.4% vs 47.2%, p = 0.01). Obesity (78.5% vs 39.4%, p < 0.001) and hypertension (51.4% vs 27.8%, p < 0.01) were more prevalent in the OSA group. IAB was more prevalent in patients with moderate-severe OSA (34.7% OSA vs 0% controls, p < 0.001). In linear regression, age and AHI > 30 were independent predictors of maximum P-wave duration (p = 0.001 and p < 0.001, respectively). P-wave dispersion was significantly higher in the severe OSA group (14.6 ± 7.5 for OSA, 8.9 ± 3.1 controls, p < 0.001)., Conclusions: Older age and moderate-severe OSA are predictors of IAB. P-wave dispersion is increased in patients with moderate-severe OSA. This may partly explain the high prevalence of atrial arrhythmias in patients with OSA.