Introduction: Left ventricular thrombus (LVT) formation following anterior ST-elevation myocardial infarction (STEMI), is a serious complication and is associated with an increased risk of cardiovascular morbidity and mortality. Recently introduced the systemic immune-inflammation index (SII) is a reliable indicator of adverse events in various cardiovascular conditions. Herein, we aimed to explore the role of SII in the prediction of LVT formation in anterior STEMI patients following primary percutaneous coronary intervention (PCI). Methods: Medical records of 124 anterior STEMI patients who developed LVT following primary PCI and 124 age and sex-matched anterior STEMI patients without formation of LVT following primary PCI were analyzed. For each group, patient demographics, comorbidities, laboratory parameters, ECG and echocardiographic findings were analyzed. The SII was measured using the following formula: SII= total peripheral platelets count x neutrophil/lymphocyte ratio. Results: According to our study, reduced left ventricular ejection fraction and increased SII were identified as independent predictors of LVT formation following anterior STEMI. To determine the SII index cut-off value for predicting LVT formation, the ROC curve was drawn and the best cut-off value was determined as 676.27 (AUC:0.778, 95% CI:0.699-0.858, p<0.001). Above this cut-off value, LVT formation could be predicted with a sensitivity of 66.1% and a specificity of 74.2%. Conclusion: SII is may be used as an independent predictor of LVT formation for anterior STEMI patients following primary PCI.