With the growing understanding of the role of inflammation in patients with atherosclerotic disease, studies have focused on high-sensitivity C-reactive protein (hs-CRP) and other inflammatory markers in their association with outcomes in ST-segment elevation myocardial infarction. The goal of this study was to investigate the association of the neutrophil/lymphocyte (N/L) ratio and in-hospital major adverse cardiac events (MACEs) in patients with ST-segment elevation myocardial infarction undergoing primary percutaneous coronary intervention (PCI). The association of hs-CRP and N/L ratio on admission with Thrombolysis In Myocardial Infarction (TIMI) flow grade after PCI was assessed in 418 consecutive primary patients with PCI. The N/L ratio was significantly higher in the no-reflow group (TIMI grade 0/1/2 flow, n = 158) compared to that of the normal-flow group (TIMI grade 3 flow, n = 260, 4.6 ± 1.7 vs 3.1 ± 1.9, p <0.001). In-hospital MACEs were significantly higher in patients with no reflow (23% vs 7%, p <0.001). There was a significant and positive correlation between hs-CRP and N/L ratio (r = 0.657, p <0.001). In receiver operating characteristic analysis, N/L ratio >3.3 predicted no reflow with 74% sensitivity and 83% specificity. In a multivariate regression model, N/L ratio remained an independent correlate of no reflow (odds ratio [OR] 1.54, 95% confidence interval [CI] 1.34 to 1.76, p <0.001) and in-hospital MACEs (OR 1.14, 95% CI 0.98 to 1.32, p = 0.043). The N/L ratio, an inexpensive and easily measurable laboratory variable, is independently associated with the development of no reflow and in-hospital MACEs in patients with ST-segment elevation myocardial infarction undergoing primary PCI.