Data are limited regarding the best prognostic glucose measure for patients admitted for an acute coronary event. We examined the admission fasting glucose levels among patients with acute coronary syndrome (ACS) from the University of Michigan ACS registry. The glucose levels were grouped into 3 categories (> or =70 to <100, 100 to <126, and > or =126 mg/dl). The primary outcome measures included mortality and a composite end point (stroke, recurrent infarction, and death) in hospital and at 6 months after the ACS event. Of the 1,525 patients (29% with diabetes) for whom glucose levels were available, a fasting glucose level of > or =100 mg/dl was associated with increased in-hospital mortality, after adjusting for the Global Registry of Acute Coronary Events risk score and gender. A fasting glucose level of > or =126 mg/dl in patients with no known history of diabetes was associated with in-hospital adverse events (odds ratio 3.37, 95% confidence interval 1.51 to 7.51). The fasting glucose level was associated with an increased risk of 6-month mortality among nondiabetics (odds ratio 3.03, 95% confidence interval 1.35 to 6.81 for patients with a glucose level of 100 to 125 mg/dl; and odds ratio 2.81, 95% confidence interval 1.07 to 7.36 for patients with a glucose level of > or =126 mg/dl) but not for diabetic patients. In conclusion, we observed a strong association between the admission fasting glucose level and mortality, particularly among nondiabetic patients. Whether improving the diagnosis and treatment of hyperglycemia would result in reductions in adverse events after ACS remains unclear.