ackground/Aim: This study was done to assess the relation between red cell distribution width (RDW) and coronary artery calcium score (CACS) in the diabetic patients undergoing coronary CT angiography and to assess the relation between RDW and severity of coronary artery disease. The objectives are to identify the role of red cell distribution width (RDW) in predicting coronary artery disease complexity and to evaluate the relationship between RDW and calcium score in diabetic patients. Methods: Thirty diabetic patients were subjected to; history taking, complete physical examination, laboratory investigations including RDW (SD & CV) as a routine examination in complete blood count, erythrocyte sedimentation rate (ESR), lipid profile. Then multidetector CT was done for coronary artery examination and calcium scoring. Results: higher RDW (SD and CV) were associated with higher calcium score and the presence of greater coronary complexity of coronary artery disease. Cut off value of RDW to presence of coronary artery disease was 11.85% and RDW (SD) cut off was 39.5 fl, while cut off value of RDW (CV) to predict high calcium score (more than 100 ) was 12,95% and RDW (SD) more than 40.5 fl. Conclusions: a greater baseline RDW (CV and SD) value may be used as an indicator of CAD and high calcium score.