Background: Aging is an established risk factor for contrast-induced nephropathy (CIN). However, little information is available on the incidence and clinical outcome of CIN for the elderly patients in Japan. Objectives: We determined the incidence and clinical outcome of CIN in the Japanese elderly patient. Methods: We studied 292 patients who had mild renal dysfunction (estimated glomerular filtration rate (eGFR) <60 ml/min/1.73 m2) at baseline and underwent coronary angiography. Patients were divided into two groups base on their age: the elderly group (age≥75, n=108) and the control group (age<75, n=184). CIN was defined as a 25% increase in serum creatinine or an increase in serum creatinine by>0.5 mg/dl above the baseline value at or within 2 days post procedure. Results: Patients in the elderly group had a higher incidence of CIN (14%) than those in the control group (9%). In patients who developed CIN, there was no significant difference between the two groups in baseline GFR and GFR on days 1, 2, 7 and 30 after the procedure. However, the relative increase in GFR above baseline on day 7 (-4.0±6.1 vs -8.3±8.0 ml/min P=0.096) and day 30 (1.5±9.4 vs -10.1±9.6 ml/min P=0.0017) in the elderly group was higher than that in the control group. Furthermore, death occurred in 3 patients in the elderly group (20%) whereas no patient died in the control group (P=0.092). Conclusion: Aging (age≥75) is a risk factor for CIN in Japanese. CIN in the elderly patients may be associated with prolonged renal dysfunction and poor prognosis.