Background: Implantable cardiac monitor (ICM) is an effective tool to detect atrial fibrillation (AF) in patients withembolic stroke undetermined sources (ESUS). We investigated predictive factors for AF detection in ESUS patientswith ICM implantation. Methods: A total of 29 ESUS patients who underwent ICM implantation (median 71 66-84 years and 18 males)were followed-up for a median of 253 44-570 days. Results: AF was detected in 10 patients (34.5%). The median time from ICM implantation to AF detection was 41.533.25-59.25 days and that from stroke onset to AF detection was 62.5 52.25-71.75 days. AF was detected within90 days after ICM implantation in 90% (9/10) of the study patients. Plasma brain natriuretic peptide level wassignificantly higher in patients with AF detection than in those without it (125 49.8-550.8 versus 18.2 14.1-60.0 pg/mL, p=0.007). More frequent supraventricular premature contraction (SVPC) on Holter electrocardiogram (ECG)was observed in patients with AF detection than in those without it (1.81 0.40-4.80 versus 0.04 0.02-0.13 %/day,p<0.001). Cox proportional hazards model showed that the frequent SVPCs was a significant factor for AF detection. Conclusions: The frequent SVPCs on Holter ECG is an independent predictor for covert AF in ESUS patients.