Contrast-induced nephropathy (CIN) is one of the serious side effects of contrast media. A few studies have suggested that N-acetylcysteine (NAC) is effective to prevent CIN, but the efficacy remains unclear in Japanese. Therefore, we retrospectively studied the preventive effect of NAC on CIN in Sakakibara Heart Institute. Patients who had been administered NAC for the purpose of preventing CIN before coronary intervention between February 2005 and November 2006 were included in the NAC group. In addition, age- and rate of diabetes mellitus-matched controls were randomly extracted. We retrieved and analyzed patient data including demographics, NAC dosage, and serum creatinine concentrations (Scr). NAC group (n=16) showed significantly higher baseline Scr (p<0.01) and a tendency toward a lower dose of contrast media (p=0.068) compared with controls (n=48). Since the occurrence of CIN was low, there was no significant difference in the proportion of CIN between the groups (NAC: 6%, controls: 4%). NAC group trended toward a decrease in Scr after the use of contrast media, while controls increased (-0.04+/-0.25 versus +0.03+/-0.36 mg/dl, p=0.096). The multivariate analysis showed that the dosage of NAC is inversely correlated with Scr independent of baseline Scr and dosage of contrast media. Despite higher baseline Scr (i.e., high-risk with CIN) in the NAC group, the real Scr value reflected a lower trend on average. In addition, this finding suggests that a larger dose of NAC results in a lower Scr value, we consider that the NAC dosage more likely prevented CIN.