Background: New-onset atrial fibrillation (AF) after coronary artery bypass graft surgery (CABG) is associated with poor outcomes, but data on the effects of its characteristics are lacking and conflicting. We examined the effect number of post-CABG AF events has on long-term mortality risk, and whether this is sex dependent. Methods: Routinely collected Society of Thoracic Surgeons (STS) data were supplemented with details on new-onset post-CABG AF (detected in-hospital by continuous electrocardiogram/telemetry monitoring) and long-term survival for 9203 consecutive patients with isolated-CABG (2002-2010). With the use of Cox regression, we determined the propensity-adjusted (STS-recognized risk factors) effect of number of AF events on survival, testing for effect modification by sex and controlling for AF duration. Results: AF occurred in 739 women (29.4%) and 2157 men (32.3%) (P < .001). Adjusted results showed 2 or more AF events significantly (P < .001) increased 5-year mortality risk, independently of total AF duration. However, mortality risk differed between the sexes (P < .001): women with 2 AF episodes had the greatest increase (hazard ratio [HR] = 2.98; 95% confidence interval [CI], 1.43-4.83; versus women without AF), followed by women and men with 4 or more AF events (HR = 2.76 [95% CI, 1.27-4.55] and HR = 2.73 [95% CI, 2.30-3.19], respectively). A single post-CABG AF episode was not associated with increased mortality risk. Conclusions: Both men and women who experienced 2 or more post-CABG AF episodes showed increased risk of 5-year mortality, independent of total AF duration. Although men's risk increased as the number of AF events increased, women's risk peaked at 2 AF events. Future research needs to determine whether this divergence stems from differences in treatment/management or underlying biology.