Background: The complete blood count is the most available laboratory test at the time of primary percutaneous coronary intervention (PCI) for ST-elevation myocardial infarction (STEMI). We hypothesized that coexistence of preprocedural anemia and high neutrophil to lymphocyte ratio (N/L) predicts poor clinical outcome in patients with STEMI treated with primary PCI. Methods: We analyzed 801 consecutive patients (63 짹 13 years, male 74 %) with STEMI treated with primary PCI within 12 hours of onset of symptoms in a single university center, from November 2005 to June 2009. Patients with cardiogenic shock or underlying malignancy were excluded, and 739 patients were included in the final analysis. The complete blood count test was performed immediately on admission. Patients were categorized into three groups by using the median value of N/L (3.86) and the presence of anemia (hemoglobin < 13 g/dl in men and <12 g/dl in women): group I, low N/L & no anemia (n=272); group II, no group I or III (n=331); group III, high N/L & anemia (n=136). Results: During the 6 month follow-up, we registered 51 composite major cardiac events at 1 month (7.0%) and 45 deaths at 6 months (6.2%). Group III were older, were more likely to have hypertension, and had worse renal function. The clinical outcome at 1 month and 6 months after PCI showed the followings: 3%, 7% and 14% of 1 month composite major cardiac events (p<0.001) and 1%, 7% and 14% of 6 month deaths (p<0.001) occurred from group 1 to 3, respectively. In a Cox proportional hazard model, after adjusting for standard risk factors, group III had higher mortality risk at 6 month (hazard ratio 7.6, 95% confidence interval 1.6 to 36.2; p = 0.011) compared with group I. Subgroup analysis of diabetic patients showed remarkable difference in the mortality rate among groups: 1.6% in group I, 8% in group II and 24% in group III (p=0.001). Conclusions: Coexistence of preprocedural anemia and high N/L indicated poor short-term clinical outcome independently after primary PCI for STEMI. Patients with preprocedural anemia and high N/L, especially in diabetic group, require more thoughtful interventional or medical approaches at the time of primary PCI for STEMI.