Introduction: Leukocytosis, predominantly neutrophilia, has previously been described following ST elevation myocardial infarction (STEMI). The exact contribution of this phenomenon to the clinical outcome of STEMI is yet to be shown. We examined cellular inflammatory response to STEMI in the blood and its association with in-hospital mortality and/or adverse clinical events.Methods: In this cross-sectional study, 404 patients who were admitted with the diagnosis of acute STEMI at Madani Heart Hospital from March 2010 to March 2012 were studied. The complete blood cell count (CBC) was obtained from all patientswithin12-24 hours of the onset of symptoms. Total leukocytes were counted and differential count was obtained for neutrophils, lymphocytes and neutrophil/lymphocyte ratio (NLR) were evaluated. Association of cellular response with the incidence of post-MI mortality/complications was assessed by multiple logistic regression analyses.Results: In-hospital mortality and post-STEMI complication rate were 3.7% and 43.6%, respectively. Higher age (P=0.04), female gender (0.002), lower ejection fraction (P<0.001) and absolute neutrophil count (P=0.04) were predictors of mortality. Pump failure in the form of acute pulmonary edema or cardiogenic shock occurred in 35 (8.9%) of patients. Higher leukocyte (P<0.03) and neutrophil counts (P<0.03) and higher NLR (P=0.01) were predictors of failure. The frequency of ventricular tachyarrhythmias (VT/VF) at the first day was associated with higher neutrophil count (P<0.001) and higher NLR level (P<0.001). In multivariate analysis neutrophil count was an independent predictor of mortality (OR=2.94; 1.1-8.4, P=0.04), and neutrophil count [OR=1.1, CI (1.01-1.20), P=0.02], female gender [OR=2.34, CI (1.02-4.88), P=0.04] and diabetes [OR=2.52, CI (1.21-5.2), P=0.003] were independent predictors of heart failure.Conclusion: A single CBC analysis may help to identify STEMI patients at risk for mortality and heart failure, and total neutrophil count is the most valuable in predicting both.