Introduction Coagulation factor IX (FIX) is considered a prothrombotic risk factor. Its increased plasma levels and activity are associated with a significantly increased risk of arterial and/or venous thrombosis. The indicator has been poorly studied in atrial fibrillation (AF).Aim To examine FIX as indicator of coagulation activity in the early hours (up to 24 hours) of the clinical manifestation of paroxysmal atrial fibrillation (PAF).Material and methods 51 non-anticoagulated patients (26 men, 25 women, mean age 59.84±1.60) with PAF and 52 volunteer controls (26 men, 26 women, mean age 59.50±1.46 years) matched in gender, comorbidities, and treatment were selected for the study. FIX plasma coagulation activity was examined using a photometric test.Results All patients were hospitalized between the 2nd and 24th hours after the arrhythmia onset (mean 8.14±0.74 hours). PAF group showed higher FIX activity relative to the control group (170.43%±6.62% vs 117.72%±5.95%, p<0.001), independent of age and BMI (p>0.05), and slightly dependent on PAF duration (r=0.38, p<0.05). Male gender (186.82%±8.98% vs 154.70%±8.38%, p<0.05) and high embolic risk (CHA2DS2-VASc score≥2) (156.03%±8.62 vs 184.31%±8.99%, p<0.05) were associated with higher FIX activity. ConclusionFIX activity is elevated even in the first twenty-four hours of the clinical manifestation of PAF. This gives reason to believe that the intrinsic coagulation cascade pathway is activated, which determines the early expression of PAF as a procoagulant state. At the same time, it is a prerequisite for FIX to become a new target for AF anticoagulant therapy with good antithrombotic efficacy and reduced bleeding risk.