Background ASSERT demonstrated that subclinical atrial fibrillation (SCAF) is common in pacemaker patients without prior AF and is associated with increased risk of ischemic stroke or systemic embolism. SCAF episodes vary in duration and little is known about the incidence of different durations of SCAF, or their prognosis. Methods and results ASSERT followed 2580 patients receiving a pacemaker or ICD, aged >65 years with hypertension, without prior AF. The effect of SCAF duration on subsequent risk of ischemic stroke or embolism was evaluated with time-dependent covariate Cox models. Patients in whom the longest SCAF was ≤6 min were excluded from the analysis (n=125). Among 2455 patients during mean follow-up of 2.5 years, the longest single episode of SCAF lasted >6 min to 6 h in 462 patients (18.8%), >6–24 h in 169 (6.9%), and >24 h in 262 (10.7%). SCAF duration >24 h was associated with a significant increased risk of subsequent stroke or systemic embolism (adjusted hazard ratio [HR] 3.24, 95% confidence interval [CI] 1.51–6.95, P=0.003). The risk of ischemic stroke or systemic embolism in patients with SCAF between 6 min and 24 h was not significantly different from patients without SCAF. Conclusions SCAF >24 h is associated with an increased risk of ischemic stroke or systemic embolism.