BACKGROUND Atrial fibrillation is known very frequent cause of ischemic stroke. Undetected paroxysmal atrial fibrillation (PAF) is thus often considered a possible cause of cryptogenic ischemic stroke (CIS). The aim of this prospective study was to detect PAF using ECG Holter monitoring and determinate whether prolongation of the Holter monitoring to 3 weeks would increase the detection rates of PAF in young CIS patients ≤ 50 years. METHODS The study set consisted of IS patients ≤ 50 years enrolled in the HISTORY (Heart and Ischemic STrOke Relationship studY) study (NCT01541163). CIS was defined according to the TOAST criteria including the absence of ultrasonographic or angiographic signs of atherosclerosis, vasculitis or dissection. Admission ECG, serum levels of high sensitive Troponin T (hs TnT) and N-terminal pro-brain natriuretic peptide (NT-proBNP), markers of thrombophilia, transoesophageal echocardiography (TEE) and 24-hour ECG-Holter monitoring were performed in all patients. In case of negative 24-h ECG Holter, an additional 3-weeks monitoring was done. RESULTS Of the 105 enrolled patients ≤ 50 years, 95 (90%) were identified as cryptogenic (49 males, mean age 39.1 ± 8.2 years). All CIS patients had normal admission ECG. In total, PAF was detected in 9 (9.5%, 95% CI: 3.5% - 17.8%) patients; in two during 24-h ECG Holter and in seven during 3-weeks Holter monitoring. Patients with PAF had more frequently elevated admission hs TnT and NT-proBNP levels (P - 0.0001). CONCLUSIONS PAF was detected in 9.5% of young CIS patients and 3-weeks ECG Holter monitoring increased the detection rate.