The neutrophil/lymphocyte ratio (NLR) has recently been described as a predictor of mortality in patients who undergo percutaneous coronary intervention. The aim of this study was to investigate the utility of admission NLRs in predicting outcomes in patients with acute coronary syndromes (ACS). A total of 2,833 patients admitted to the University of Michigan Health System with diagnoses of ACS from December 1998 to October 2004 were followed. Patients were divided into tertiles according to NLR. The primary end point was all-cause in-hospital and 6-month mortality. The ACS cohort comprised 564 patients with ST-segment elevation myocardial infarctions and 2,269 patients with non-ST-segment elevation ACS. Patients in tertile 3 had higher in-hospital (8.5% vs 1.8%) and 6-month (11.5% vs 2.5%) mortality compared with those in tertile 1 (p <0.001). After adjusting for Global Registry of Acute Coronary Events risk profile, patients in the highest tertile were at an exaggerated risk for in-hospital (odds ratio 2.04, p = 0.013) and 6-month (odds ratio 3.88, p <0.001) mortality. Admission NLR is an independent predictor of in-hospital and 6-month mortality in patients with ACS. This relatively inexpensive marker of inflammation can aid in the risk stratification and prognosis of patients diagnosed with ACS.