Background Acute hyperglycemia is a common finding in both diabetic and non-diabetic patients with acute coronary syndrome (ACS) who present to the emergency department (ED). The prognostic role of hyperglycemia in diabetic patients with ACS remains controversial at least on the short-term basis. The aim of the present study was to find the relation between the glycemic gap and clinical outcome in diabetic patients with ACS. Methods The study included 100 diabetic patients with ACS to who were prospectively followed during their hospital stay. Admission blood glucose was measured and glycemic gap was calculated. Results In diabetic patients suffering ACS, there was a significant relation between the glycemic gap value, occurrence of major adverse cardiovascular events (MACE) and length of hospital stay. Conclusion Glycemic gap is a better marker than admission blood glucose alone in diabetic patients presenting with ACS. This study added the optimal cut-off value for this important biomarker.