Background: Diabetes mellitus (DM) ranks as one of the most potent risk factors of cardiovascular diseases. There is a strong body of evidence that links chronic hyperglycemia to microvascular complications. However, the available results regarding its correlation with macrovascular complications are conflicting. The purpose of this study is to determine the correlation between the level of Glycated hemoglobin (HbA1c), as a marker of chronic hyperglycemia, and the severity of coronary artery disease.Study design and methods: The study included five hundred and ten patients referred to coronary angiography in four tertiary centers. In addition to the routine evaluation, HbA1c level, transthoracic echocardiogram and coronary angiogragraphy were done and the Gensini score was calculated. Diabetic patients were classified into controlled and uncontrolled based on the cutoff point of HbA1c value 7%. While non-diabetic patients were classified as low and high risk groups for development of DM. Results: Three hundred and twenty eight patients (64%) had DM and 57.5% were hypertensive. 71% of the diabetic group had HbA1c ≥ 7 mg% with mean HbA1c of 8.6 ±2.2. The mean Gensini score was 35 ±30. There was significant positive correlation between the level of HbA1c and duration of DM (r = 0.456, p < 0.000), BMI (r = 0.183, p < 0.000), RWMSI (r = 0.109, p = 0.014) and Gensini score (r = 0.197, p = 0.008) (among the non-diabetic group). However, among the diabetic group there was no significant correlation with Gensini score (r =0.089, p =0.109). There was positive correlation between Gensini score and RWMSI (r = 0.413, p <0.000) and negative correlation between Gensini score and LVEF (r = -0.364, p <0.000).Conclusion: HbA1c level is a useful biomarker and has a prognostic value to predict the severity of CAD among non-diabetic patients. In diabetic patients, however, HbA1c was not a good predictor of the severity of CAD.