BACKGROUND: Although factor XI (FXI) inhibitors are currently tested for the prevention of thrombotic events, their early treatment could prevent thrombus consolidation in ST-segment elevation myocardial infarction (STEMI). This study aims to characterize coagulation FXI levels and their variations in patients with STEMI undergoing primary percutaneous coronary intervention (PCI). METHODS: Patients with STEMI were prospectively enrolled between December 2023 and May 2024. FXI activity (FXIa) levels were measured at admission and after PCI (i.e., before discharge). Variations in FXIa levels were evaluated. Differences in indicators of thrombotic risk between groups with high and low FXIa variability were analyzed, and predictors of high FXIa variability were identified. RESULTS: After screening, 54 patients with STEMI were included. The median FXIa level was 0.865 IU/mL (interquartile range [IQR] 0.554-0.978) at admission and 1.161 IU/mL (IQR 0.982-1.317) before discharge, with a median difference of +34.2% ( CONCLUSION: This study first reported an increase in FXIa levels from admission to discharge in STEMI patients undergoing PCI. Common indicators of thrombotic risk were not associated with FXIa levels or their variability. These findings aim to stimulate further research into anticoagulant therapies tailored to the patient's coagulative state and disease.