Objective. The systemic immune inflammatory index (SII) has prognostic value in cardiovascular diseases. The aim of current study was to investigate whether or not left atrial appendage (LAA) thrombus could be predicted by SII in patients with non-valvular atrial fibrillation. Method. The study included 525 patients newly diagnosed with non-valvular atrial fibrillation, who had not previously had anticoagulant treatment (50.7% male, mean age 62.94±10.79 years). All patients underwent transoesophageal echocardiography. Results. LAA thrombus was observed in 86 patients (16.4%). In the ROC curve SII had a good diagnostic power in predicting LAA thrombus (AUC: 0.760, 95% CI: 0.703-0.818, P<0.001). In the multivariate regression analysis, diabetes (