Background Patients with atrial fibrillation (AF) have a high incidence of cognitive impairment, which may be related to clinically silent microembolism causing cerebral infarctions. Objective To explore the occurrence and timing of silent brain lesions following electrical cardioversion (CV) of recent onset AFin anticoagulant-naïve patients and to study related effects on cognitive function and biomarkers of cerebral damage, S100b. Methods Patients with AF duration < 48 hours were prospectively included. Brain magnetic resonance imaging (MRI) and S100b, were obtained prior, after and 7-10 days following CV. Trail making tests(TMT-A and TMT-B)and their difference, ΔΤΜΤ, were assessed prior to CV, 7-10 days and30days after CV. Results Forty-three patients (84%males) with medianCHA2DS2-VASc score 1(interquartile range 0-1) were included. Sequential MRI, including diffusion weighted scans, showed no new brain lesions after CV. Chronic white matter hyperintensities were present at baseline in 21/43 (49%) patients. The S100b (µg/l) levels increased significantly from baseline, (mean±SD)0.0472±0.0182to 0.0551±0.0185 after CV, p=0.001 andthen decreased 7-10 days after CV to 0.0450±0.0186, p<0.001. Consecutive TMT scores improved successively after CV, being statistically and clinically significant for TMT-B (p<0.01) andΔΤΜΤ (p=0.005) between 7-10 days and 30 days after CV (Reliable Change Index >1.96). Conclusion New brain lesions could not be detectedon MRI after CV, but the high incidence of white matter hyperintensities and the transient increase in S100b may indicate transient or minor brain damage undetectable by MRI thus heightening the need to reevaluate thromboembolic risk prior to CV even in low risk patients.