Background and objective: Secondary prevention after embolic stroke of undetermined source (ESUS) remains a clinical problem. Presence of asymptomatic cerebral large-artery atherosclerosis or small vessel disease could be aprioristically taken as an indicator of a lower risk for an occult cardiac source of emboli, thus influencing our secondary prevention strategy. We aimed to study the relationship between presence and degree of coexisting cerebrovascular disease and the risk of occult paroxysmal atrial fibrillation (OPAF) in ESUS patients. Methods: Longitudinal prospective study in patients fulfilling ESUS criteria after complete neurovascular and cardiac diagnostic workup, who were implanted with a subcutaneous REVEAL-XT loop-recorder to detect OPAF and followed-up for≥ 6 months. At baseline, cerebral large-artery atherosclerosis was assessed with cervical and transcranial ultrasound. Brain magnetic resonance imaging was used to evaluate small vessel disease. Periventricular (PV) and subcortical (SC) white matter hiperintensities (WMH) were categorized using the Fazekas score. Results: We studied 136 ESUS patients from October 2010 to December 2013 (71 men, mean age 67), who were followed-up for a mean time of 594 days. OPAF was detected in 56 (41%) of them. No relationship was found between extracranial or intracranial atherosclerosis and OPAF. Kaplan-Meier curves and crude Cox-regression analyses found associations between OPAF risk and age, smoking, CHA2DS2VASC score, presence of lacunar infarctions, and presence and degree of PVWMH & SCWMH. A multivariable-adjusted Cox regression model identified grade 2-3 PVWMH (HR 3.6, [2.0-6.5], p<0.001) and age as independent predictors of OPAF. Conclusion: Coexisting small vessel disease, specifically in the form of periventricular WMH, is a predictor of OPAF in ESUS patients. Presence of large-artery atherosclerosis does not lower the risk for OPAF. Therefore, OPAF should be actively pursued in ESUS patients regardless the coexistence of asymptomatic cerebrovascular disease.