Background: Monocyte to high-density-lipoprotein cholesterol ratio (MHR) simply reflects proatherogenic and antiatherogenic balance and high level of this ratio is associated with severity of coronary atherosclerosis and cardiac events. We investigated the association between MHR and coronary artery disease severity using SYNTAX score and SYNTAX score II (SSII) in ST-elevation myocardial infarction (STEMI) patients treated with primary percutaneous coronary intervention (pPCI). Methods: A total of 315 consecutive patients with STEMI who underwent pPCI fromJanuary 2014 to January 2016 were enrolled. After exclusion 264 patients remained in the study population. Patients were divided into 2 groups according to median SSII [SSII  34.2 as low group (n = 132) and > 34.2 as high group (n = 132)]. Results: Median value ofMHR was 10.5 in SSII low group and 16.1 in SSII high group (p < 0.001). There was a strong correlation between MHR and SSII (r = 0.580, p < 0.001). Diabetesmellitus [odds ratio (OR): 8.604; 95% confidence interval (CI): 2.469-29.978], glomerular filtration rate (OR: 0.961; 95% CI: 0.939-0.983), infarct related artery of left anterior descending (LAD) (OR: 7.325; 95% CI: 2.262-23.723), SYNTAX score (OR: 1.422; 95% CI: 1.275-1.585), neutrophil to lymphocyte ratio (NLR) (OR: 1.156; 95% CI: 1.058-1.264) and MHR (OR: 1.027; 95% CI: 1.013-1.041) were independent predictors of SSII > 34.2 in multivariate analysis. Conclusions: MHR could be a better parameter than NLR and C-reactive protein at predicting severity of coronary artery disease in STEMI patients treated with pPCI.