Background Although the relationship of either hemoglobin or red blood cell distribution width (RDW) with contrast-induced nephropathy (CIN) has been reported individually. To date, no studies have evaluated the predictive value of hemoglobin-to-red blood cell distribution width ratio (HRR) for CIN. Methods A total of 1658 elderly patients with ST-segment elevation myocardial infarction (STEMI) undergoing emergency percutaneous coronary intervention (PCI) were retrospectively screened. Preoperative complete blood count was collected and the HRR was calculated as the ratio of hemoglobin to RDW. CIN was defined as an absolute >= 0.5 mg/dL (44.2 mu mol/L) or a relative >= 25% increase in creatinine level at 72 h after contrast administration. Univariate and multivariate regression analysis were conducted to determine the effective predictors for CIN. The ROC curve analysis was plotted to determine the optimal cutoff value for HRR in predicting CIN. Results The overall incidence of CIN was 8.38%. The HRR was significantly lower in the CIN group compared with the non-CIN group (0.87 +/- 0.15 vs 1.24 +/- 0.23, p < 0.001). After multivariate regression analysis was performed, HRR was noted to be an effective predictor for the development of CIN (OR 1.617, 95% CI 1.439-2.706, p = 0.014), along with age, creatinine, eGFR, hs-CRP and contrast volume. An optimal cutoff value of 0.94 or lower for HRR was identified with 82.4% sensitivity and 63.5% specificity to predict CIN. Conclusion Lower HRR on admission was an effective predictor for CIN in elderly patients with STEMI undergoing emergency PCI. HRR may be a convenient, economical and reliable biomarker for risk stratification.