Abstract Funding Acknowledgements Type of funding sources: None. Background The relationship between the excessive supraventricular ectopic activity (ESVEA) and the subclinical atrial fibrillation (AF) in patients with cryptogenic stroke is yet not fully understood. Purpose The aim of this study is to examine the prognostic significance of ESVEA for the development of AF in those patients. Methods The study retrospectively included 124patients, hospitalized for a cryptogenic stroke between 2014 and 2015. Twenty-four hour inpatient Holter monitoring, was used to define ESVEA as the presence of ≥20 premature atrial complexes per hour (PACs/h), along with a duration of the longest run of supraventricular tachycardia (LSVR) ≥5 seconds. After approximately 5 years of follow-up, the patients were examined for AF. Results The remaining 111 patients(12 died and 1 was lost during follow-up) had a median age of 56 and 13 (11.71%) of them were diagnosed with AF (AF patients). The median value of CHA2DS2-VASc score was 3 and was similar for the two groups (p=0.252). Patients with AF had a significantly higher number of PACs/h and a longer duration of LSVR compared to nonAF patients (16.67 vs. 0.21, p<0.001 and 3 vs. 0 seconds, p<0.001, respectively). The existence of ESVEA was also significantly more prevalent among the AF patients (46.15%, 95% CI: 17.78% - 74.22%) compared to non-AF ones (6.1%, 95% CI: 1.3% - 10.7%, p<0.001). ROC analysis revealed the high diagnostic test accuracy of both PACs/h and LSVR for AF. The area under the curve was 97.2% (p<0.001) for PACs/h and 81.1% (p<0.001) for LSVR. Conclusions Excessive atrial ectopy, detected with 24h inpatient Holter monitoring, is a significant indicator of future development of AF, in patients presenting originally with a cryptogenic stroke.