Diabetes is an established major factor of poor prognostis after an acute coronary syndrome. Recent studies have addressed the impact of abnormal glucose metabolism at the acute phase in patients without known diabetes. It has been found that abnormal glycemia regulation is more common than normal regulation in patients presenting with acute coronary syndrome, whatever the method used to evaluate blood glucose metabolism. High blood glucose at admission, whether fasting or not, are associated with worse outcome after an acute coronary syndrome, ie. by increased mortality and development of severe heart failure. The prognosistic value of glycemia is valuable for both short and long term outcomes. Admission glycemia measurement allows therapeutic strategies at the acute phase. Fasting glycemia and oral glucose tolerance test performed during the hospital stay discloses valuable diagnostic information and provide useful tools for secondary prevention. Moreover, fasting glycemia is a more powerful predictor for short term outcome after myocardial infarction than admission glycemia. The mechanisms by which hyperglycemia deteriorates the cardiovascular prognosis, in particular for left ventricular dysfunction, are not fully understood. Stress hyperglycemia may be a marker of extensive cardiac damage, reflecting a surge of stress hormones such as catecholamines and cortisol that participate to insulinresistance and affect fatty acid and glucose homeostasis. Recent findings also argue for a direct deleterious effect of hyperglycemia on myocardium.