Backround: Spontaneous echo contrast (SEC) is an echocardiographic finding particularly found in left atrium of patients with mitral stenosis (MS) and known as a risk factor for stroke. However, its pathophysiology is not fully understood. Methods: Forty-eight patients with MS scheduled for percutaneous mitral valvuloplasty were included in the study. Blood samples were taken from the aorta and left atrium (LA) during the procedure. Whole blood viscosity (WBV), plasma viscosity (PV) and peripheral blood smears were obtained and analysed separately from these sites. All participants underwent transthoracic and transesophageal echocardiography prior to the procedure Results: Severe SEC (grade 3-4) was found in 23 patients, remaining 25 patients had mild to moderate SEC (grade 0-1-2). Patients with severe SEC had increased LA diameter, area and PV. However, ejection fraction, left atrial appendage (LAA) filling and emptying velocities, LAA lateral wall late systolic velocity, LAA fractional area change and pulmonary vein (PVe) systolic velocity were found to be significantly reduced in patients with severe SEC compared to mild to moderate SEC. On multiple linear regression analysis, atrial fibrillation, left atrium PV and diameter were strongly correlated with SEC grade (Respectively p=0,011, p=0,013, p=0,030). Conclusion: We have shown that AF, systolic dysfunction of LAA and left ventricule, reduced PVe flow velocity, increased LA dimensions and left atrial PV were related with the severity of SEC in patients with mitral stenosis. We demonstrated the relationship between the increase left atrial PV and SEC in addition to impaired hemodynamic determinants in patients with mitral stenosis.