Abstract: AIM To investigate the association between the nertrophil-to-lymphocyte ratio (NLR) and severity of acute coronary syndrome. METHODS Two hundred and sixty-four patients with coronary heart disease, including 122 with unstable angina (UA) and 132 with acute myocardial infarction (AMI), as well as 53 coronary angiography negative patients (control group) were included in this study. WBC count was detected and NLR was calculated. The relationship between severity of acute coronary syndrome and clinical data, laboratory test results, NLR, and other inflammatory markers were examined. RESULTS Compared with the control group, NLR, neutrophil count was significantly increased in the UAP group, the difference was statistically significant (P<0.05). In the AMI group, white blood cell count, neutrophil count, and NLR significantly increased, the difference was statistically significant (P<0.05); Multivariate logistic regression analysis showed that NLR (OR 9.398,95% CI 1.352-3.934, P<0.05) was an independent risk factor in ACS. The levels of NLR and Gensini integral were positively correlated. CONCLUSION NLR is associated with coronary heart disease and severity of the disease.