Objective: Inflammation is a known pathophysiological feature that is common in both diabetes mellitus (DM) and atherosclerosis. High-density lipoprotein (HDL) is an anti-atherogenic lipoprotein and has an inverse relationship with coronary artery disease (CAD). In the present study, the association between the severity of CAD and inflammatory parameters neutrophil to HDL (NHR), lymphocyte to HDL (LHR), and platelet to HDL (PHR) was investigated. Methods: In this single-center prospective study, 98 diabetic (type 2 DM), and 320 non-diabetic CAD patients who underwent coronary angiography were enrolled. Baseline demographic characteristics were recorded, hematologic and biochemical samples were measured, the angiographic profile was analyzed, SYNTAX and Gensini scores were calculated for all patients. Results: Hypertension and smoking rates and HbA1c were higher in the diabetic group (p:0.001, p<0.001, p<0.001, respectively). NHR, LHR, and PHR were significantly higher in the diabetic group compared with the non-diabetic group (p:0.007, p:0.002, p:0.005, respectively). The diabetic group had higher SYNTAX and Gensini scores (p<0.001 for both). NHR, LHR, PHR, age, and hemoglobin were positively correlated with both SYNTAX and Gensini scores in the correlation analysis. Univariate and multivariate analyses showed that NHR, LHR, SYNTAX and Gensini scores were found to be independent predictors of DM in CAD patients. Conclusion: Our study demonstrated that the inflammatory parameters NHR, LHR, and PHR and atherosclerotic burden parameters SYNTAX and Gensini scores were worsening in diabetic patients. NHR, LHR, and PHR were significantly associated with higher SYNTAX and Gensini scores in diabetic and nondiabetic patients.