Aims: Recently, a number of studies have shown an increased red blood cell distribution width (RDW) to be a strong and independent predictor of the prognosis of coronary artery disease. The aim of this study was to investigate the underlying mechanisms responsible for the relationship between the RDW and a poor prognosis of coronary artery disease. Methods: Four hundred and twenty-four patients with ST elevation myocardial infarction treated with primary percutaneous coronary intervention (pPCI) were analyzed retrospectively. We evaluated the relationships between the RDW and the high-sensitivity C-reactive protein (hsCRP) N-terminal pro-brain natriuretic peptide (NTpro-BNP), fasting blood glucose and lipid levels, as well as other parameters of blood examinations and angiographic manifestations. Results: There were 85 patients in the RDW >= 14% group (mean age 60.62 +/- 11.29 years, and men: 87%) and 339 patients in the RDW < 14% group (mean age: 59.74 +/- 11.55 years, and men: 78%). The RDW >= 14% group had higher platelet distribution width (PDW), NTpro-BNP and hsCRP values on admission, a heavier intracoronary thrombotic burden and a higher incidence of three-branch vascular lesions than the RDW < 14% group. In the multiple logistic regression analysis, the associations between the RDW and the NTpro-BNP level, incidence of three-branch and left main lesions and intracoronary thrombotic burden remained. Conclusions: A high RDW may be associated with the severity and instability of acute myocardial infarction.