Background: In the Mode Selection Trial (MOST) of 2,010 patients with sinus node dysfunction, dual-chamber-paced patients had less atrial fibrillation (AF) and heart failure and had slightly improved health-related quality of life (QOL) compared with rate modulated right ventricular-paced patients. Our objective was to assess the impact of AF on QOL within MOST. Methods: We analyzed serial QOL measures (Short Form-36, Specific Activity Scale, time trade-off) in 3 groups: (1) those without AF; (2) those with paroxysmal AF (PAF), but not chronic AF (CAF); and (3) those with CAF. We carried forward the last known QOL before crossover for all subsequent time points in patients randomized to rate modulated right ventricular pacing who crossed over to dual-chamber pacing for severe pacemaker syndrome. Results: Three hundred seventeen patients (15.8%) had AF in the year after implantation, 206 patients within 3 months (191 PAF, 15 CAF), and another 159 (124 PAF, 35 CAF) between 3 and 12 months. There were no significant differences among groups in individual Short Form-36 subscales or time trade-off scores at 12 months as compared with baseline or 3 months. Cardiovascular health status was better at 12 months as compared with baseline or 3 months in those without AF. Conclusions: Atrial fibrillation after pacemaker implantation in elderly patients with sick sinus syndrome was not a major determinant of QOL. However, there was a trend toward better cardiovascular functional status in patients without AF. © 2009 Mosby, Inc.