Objectives: We aimed to investigate the relationship between red cell distribution width (RDW) value and coronary collateral circulation (CCC) in patients with non-ST elevation myocardial infarction (NSTEMI). Methods: The study population consisted of 322 consecutive patients with NSTEMI. The patients were classified into impaired CCC (group 1, Rentrop grades 0-1) or good CCC (group 2, Rentrop grades 2-3). Baseline RDW was measured as part of the automated complete blood count. Results: The RDW values were significantly higher in patients with impaired CCC than in those with good CCC (17.2 ± 2.3 vs 14.5 ± 2.5, P < .001). In multivariate logistic regression analysis, RDW (odds ratio: 1.52, 95% confidence interval: 1.30-1.78, P < .001), baseline creatine kinase MB (CK-MB), and absence of preinfarction angina were found to be the independent predictors of impaired CCC. In receiver–operating characteristic curve analysis, the RDW value >15.5 yielded an area under curve value of 0.783, with 77% sensitivity and 73% specificity. Conclusions: Our study results demonstrated that, high RDW, high CK-MB, and absence of preinfarction angina were found to be independent predictors of impaired CCC.