Background: Elderly patients with impaired renal function are especially at high risk of developing contrast induced nephropathy(CIN). Although adequate hydration is known to protect patients undergoing coronary angiography(CAG) from CIN, other conditions may still place these patients at increased risk. We performed a prospective study to find predictors of CIN development in high risk elderly patients with impaired renal function undergoing CAG, who were adequately hydrated. Methods: We prospectively enrolled 258 patients over the age of 60 with baseline calculated creatinine(Cr) clearance<60 mL/min, undergoing CAG. Patients were hydrated with intravenous half-isotonic saline at a rate of 1mL/kg per hour at least 8 hours before and after CAG. Serum Cr at baseline and at day1 and day2 after CAG were measured and various risk factors of CIN were also recorded. Results: Data regarding the development of CIN was not available in 11 patients and thus 247 patients were analyzed. The incidence of CIN in this high risk population was 4.05%. On univariate analysis, patients that developed CIN had significantly lower HDL-C, hematocrit levels, and left ventricular function, and significantly higher BUN levels. In addition, there were more patients with DM, congestive heart failure (CHF: LV EF<40%), female gender, and microalbuminuria (Ma) or proteinuria (PU) in the group that developed CIN. However, statin, N-acetyl-cysteine, and ARB or ACE inhibitor use, the presence of hypertension, hyperlipidemia, hyperuricemia, hypoalbuminemia, and smoking were not associated with CIN. By multivariate analysis, female gender (OR 1.875, CI: 1.16-36.66), CHF (OR 1.902, CI: 1.31-34.24), and the presence of Ma or PU (OR 2.507, CI: 1.35-111.52) were independent predictors of CIN development. Conclusion: In high risk elderly patients with impaired renal function, adequate hydration results in a relatively low incidence of CIN compared with those reported in the literature. However, females, those with CHF, and/or baseline Ma or PU are especially at greater risk for CIN development. Our data suggest that these patients may be at higher risk of developing CIN even with adequate hydration, and may require additive reno-protective measures.