Background: Coronary Artery Disease (CAD) is the leading cause of morbidity and mortality all around the world. Red cell distribution width (RDW) is an indicator for the variability and size of circulating erythrocytes, has recently been shown to be an independent predictor of prognosis in patients with cardiovascular disease. Objectives: to assess the relationship between RDW and severity of coronary artery disease (CAD) by SYNTAX score in patients with ST-elevation myocardial infarction (STEMI) undergoing coronary angiography. Patients and methods: Eighty consecutive patients, who underwent coronary angiography after diagnosis of STEMI, were enrolled in this study which was conducted at cardiology department of Qena university hospital and Qena general hospital at period from October 2018 to July 2019. Results: there was no statistical significant difference (p-value > 0.05) between normal RDW patients and Abnormal RDW patients as regard demographic data(age, sex, BMI, smoking, HTN, dyslipidemia) except for DM, there was statistical significant difference, also there was no statistical significant difference (p-value > 0.05) between normal RDW patients and Abnormal RDW patients as regard laboratory data (eGFR,WBCs, neutrophils, lymphocytes, N/L ratio). While there was statistically significant difference (p-value < 0.05) between normal RDW patients and Abnormal RDW patients as regard LVEF, SWMA, number of affected vessels and SYNTAX score. After adjusting for all correlates, patients was high syntax scores were 3.6 times more liable for having abnormal RDW class (AOR=3.6, 95% CI: 1.2-7.3, p-value =0.029). Conclusion:Red cell distribution width is positively correlated with number of diseased vessels and high syntax score and extent of coronary artery disease.