Background: Neutrophil/lymphocyte ratio (NLR) is a novel biomarker that can single out individuals at risk for vascular events. We assessed whether NLR provides additive prognostic value in patients with ST-elevation myocardial infarction (STEMI). Methods: NLR was computed from the absolute values of neutrophils and lymphocytes from the complete blood count of patients who underwent primary coronary angioplasty for STEMI. The cohort was divided into 2 groups according to NLR (NLR ≥ 6.5%, NLR < 6.5%) using χ2 automatic interaction detection. The association between NLR and in-hospital clinical complications and left ventricular ejection fraction (EF) was assessed using logistic regression. The association between NLR, 30-day and 5-year all-cause mortality were analyzed using Cox regression models, adjusting for potential clinical, metabolic, and inflammatory confounders. Results: In a group of 538 consecutive STEMI patients, high NLR (NLR ≥ 6.5%) was independently associated with increased 30-day and 5-years mortality rates (odds ratio, 15.8; 95% confidence interval, 1.6-154; P= 0.018; and hazard ratio, 2.2; 95% confidence interval, 1.04-4.8; P= 0.039, respectively). High NLR was also independently associated with lower EF (49 ± 8 vs 46 ± 8; P < 0.001) and fewer hospital complications. Conclusions: In patients presenting with STEMI, high NLR was independently associated with lower EF, fewer hospital complications, and higher mortality rates up to 5 years. NLR value appears additive to conventional risk factors and commonly used biomarkers.