Background: The nephrotoxicity induced by contrast media remains a serious clinical problem, and the underlying mechanism has not been completely understood. Experimental and clinical investigations suggest that reactive oxygen species (ROS) are critical determinants of radiocontrast nephropathy (RCN), and that antioxidants can prevent this damage. Methods: Cultured human proximal renal tubule cells (HK-2) were exposed to hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) at different concentrations. H<sub>2</sub>O<sub>2</sub>-induced tubular DNA damage was examined in the presence of the antioxidant MESNA (sodium-2-mercaptoethane sulphonate). The induction of DNA damage was measured with the alkaline comet assay (single cell gel electrophoresis). We also studied 12 patients with stable renal impairment (median baseline creatinine 296 µmol/l; range: 203–495 µmol/l) undergoing cardiac catheterization/intervention prospectively. Patients received 800 mg MESNA intravenously 30 min before exposure to the contrast agent in addition to 0.9% saline hydration. Results: In the cell cultures, oxidative stress on HK-2 cells induced increased DNA migration in the comet assay. Treatment of tubular cells with the antioxidant MESNA prior to the addition of H<sub>2</sub>O<sub>2</sub> significantly reduced DNA migration in the comet assay. In the clinical study, treatment of the patients with MESNA prevented the adverse renal effect of contrast media (median serum creatinine 293; range: 187–433 µmol/l) 48 h after coronary angiography/intervention. Conclusion: Both the in vivo and the in vitro studies suggest that the ROS-mediated renal injury could be inhibited by a potent antioxidant such as MESNA.