Systemic immune-inflammation index (SII; platelet count x neutrophil-to-lymphocyte ratio), a novel marker, predicts adverse clinical outcomes in coronary artery diseases (CAD). We hypothesized that SII could provide more valuable information in assessing the severity of CAD than ratios obtained from other white blood cell subtypes. Patients (n = 669) who underwent coronary angiography were analyzed in this retrospective study. We analyzed the relation between the SII and the angiographic severity of CAD. The severity of coronary atherosclerosis was determined by the SYNTAX score (SxS). Patients with CAD were divided into 3 groups according to the SxS. Multivariate logistic analysis was used to assess risk factors of CAD. In multivariate logistic regression analysis, the SII (odds ratio: 1.004; 95% CI: 1.001-1.007; P = .015) was an independent predictor of high SxS. Additionally, there was a positive correlation between SII and SxS (Rho: 0.630, P <= .001). In the receiver-operating characteristic curve analysis, SII with an optimal cutoff value of 750 x 10(3) predicted the severe coronary lesion with a sensitivity of 86.2% and specificity of 87.3%. The SII, an inexpensive and easily measurable laboratory variable, was significantly associated with the severity of CAD and high SxS in patients with stable angina pectoris.