OBJECTIVE: Numerous mechanisms have been proposed for the no-reflow phenomenon (NRP) in the literature including leukocyte intravascular plugging, microembolisms, and extrinsic coagulation pathway activation. Some of the more recent studies suggested a relationship between NRP and systemic immune-inflammation index (SII) in different contexts. To this end, the objective of this study was to investigate the relationship between NRP and SII in acute coronary syndrome (ACS) patients with coronary artery bypass grafting (CABG) who underwent percutaneous transluminal coronary angioplasty (PTCA) or percutaneous coronary intervention (PCI) of saphenous vein graft (SVG). PATIENTS AND METHODS: The sample of this retrospective study consisted of 124 ACS patients with CABG who underwent PTCA/PCI of SVG. RESULTS: The incidence of NRP in the study group was 30.6% (n=38). The results of the multivariate logistic regression analysis indicated that ST-elevation myocardial infarction (STEMI) and SII were independent predictors for NRP (p<0.05). The receiver operating characteristic (ROC) curve analysis revealed that the optimal cut-off value of SII in predicting the development of NRP in patients undergoing PTCA/PCI of SVG and the sensitivity and specificity values thereof are 975, 74%, and 80%, respectively [Area under the curve (AUC): 0.84, 95% confidence interval (CI): 0.76-0.91, p-value <0.001]. CONCLUSIONS: The study findings indicated that SII, which can be easily calculated from a single complete blood count test, is an independent predictor of the development of NRP in ACS patients undergoing PTCA/PCI of the SVG.