Objective: Hyperglycemia on admission and during hospital stay is a well-established predictor of short-term and long-term mortality in patients with acute myocardial infarction. Our study investigated the impact of blood glucose levels on admission and in-hospital hyperglycemia on the morbidity and mortality of Libyan patients admitted with acute coronary syndromes (acute myocardial infarction and unstable angina). Methods: In this retrospective study, the records of patients admitted with acute coronary syndrome to The 7th Of October Hospital, Benghazi, Libya, between January 2011 and December 2011 were reviewed. The level of blood glucose on admission, and the average blood glucose during the hospital stay were recorded to determine their effects on in-hospital complications (e.g. cardiogenic shock, acute heart failure, arrhythmias, and/or heart block) and mortality. Results: During the study period, 121 patients with diabetes were admitted with acute coronary syndrome. The mortality rate in patients with diabetes and acute coronary syndrome was 12.4%. Patients with a mean glucose level greater than 200mg/dL had a higher in-hospital mortality and a higher rate of complications than those with a mean glucose level ≤200mg/dL (27.5% vs. 2.6%, p<0.001 and 19.7% vs. 45.5%, p=0.004, respectively). There was no difference in in-hospital mortality between patients with a glucose level at admission ≤140mg/dL and those admitted with a glucose level >140mg/dL (6.9% vs. 14.3%; p=0.295), but the rate of complications was higher in the latter group (13.8% vs. 34.1%; p=0.036). Patients with admission glucose levels >140mg/dL also had a higher rate of complications at presentation (26.4% vs. 6.9%; p=0.027). Conclusion: In patients with diabetes and acute coronary syndrome, hyperglycemia during hospitalization predicted a worse outcome in terms of the rates of in-hospital complications and in-hospital mortality. Hyperglycemia at the time of admission was also associated with higher rate of complications particularly at the time of presentation.