BACKGROUND: Short-lasting (<30 s), nonsustained episodes of atrial fibrillation (NS-AF) are considered a risk factor for future development of paroxysmal or persistent AF. Nonetheless, their causal role in stroke pathogenesis is currently unknown. In this study we determined the frequency of NS-AF, together with the associated clinical and imaging features, in stroke-free controls and ischemic stroke patients. METHODS AND RESULTS: A total of 332 controls, ≥50 years of age and no prior history of stroke or AF, were evaluated with 24-hour Holter monitoring for the presence of <30-s-long AF episodes. The demographic and cardiovascular features of this cohort, together with imaging finding on magnetic resonance imaging, were compared to a consecutive series of ≥50-year-old ischemic stroke patients without AF (n=498). The prevalence of NS-AF was significantly higher among ischemic stroke patients in comparison to controls (37% versus 27%; P=0.002). In multivariable analyses, after adjustment for demographic and cardiovascular risk factors, patients with ischemic stroke were more likely to harbor NS-AF episodes (odds ratio 1.43; 95% CI 1.01-2.02; P=0.041). The association between ischemic stroke and NS-AF weakened when the analyses were restricted to cryptogenic stroke patients (odds ratio 1.31; 95% CI 0.82-2.08). No significant association was observed between the presence of chronic cortical infarcts and NS-AF. CONCLUSIONS: Our study shows a higher prevalence of NS-AF episodes in ischemic stroke patients in comparison to controls. Nonetheless, the lack of a stronger association with cryptogenic strokes and absence of a relationship with chronic cortical infarcts brings into question the causal influence of NS-AF in the ischemic stroke setting.