A role of coagulation in the pathogenesis of aortic stenosis (AS) is unknown. The aim of this study was to investigate the fibrin (Fn) presence and its determinants in calcified stenotic aortic valve leaflets. Twenty-one patients with dominant AS and 17 well-matched patients with dominant aortic insufficiency (AI) undergoing aortic valve replacement were studied. Immunofluorescence analysis was performed on decalcified leaflets using antibodies against human Fn and tissue factor (TF). Fn-positive (41.4%) and TF-positive (25.3%) areas were increased in AS valves compared with AI valves (7.9% and 5.9%, respectively, both p<0.001). Patients with AS had elevated plasma D-dimer (236.4 ± 28 ng/ml, p=0.002) and prothrombin fragment 1+2 (F1.2) (261.7 ± 27.1 pM, p=0.005) compared to AI subjects (142.8 ± 10 ng/ml and 131.2 ± 1.3 pM, respectively). In AS patients Fn-positive areas correlated with TF-positive areas (r=0.68, p=0.0005), D-dimer (r=0.45, p=0.018), F1.2 (r=0.64, p=0.002), the time required for plasma fibrin clot formation (r=0.44, p=0.015) and maximum absorbance of fibrin clots (r=-0.38, p<0.0001), but not with clot permeability or lysis time. Thickness of Fn layer within AS valves was associated with maximum transvalvular gradient (r =0.41, p=0.048). Patients with maximal gradient above 75 mmHg (n=11) showed significant associations between Fn-positive area and both maximal (r =0.63) and mean (r =0.67) transvalvular gradients. Large fibrin amounts, mostly co-localised with TF, are present within the valve leaflets of patients with advanced AS. In vivo thrombin generation and fibrin clot formation are associated with the extent of Fn presence within leaflets, which might contribute to the AS progression.