Oxidative stress may contribute to the progression of chronic renal failure. In this study, cats with spontaneous renal insufficiency were fed a dry cat food supplemented with the antioxidants vitamins E and C, and beta-carotene for 4 weeks. When compared with healthy cats, cats with renal insufficiency had a tendency to oxidative stress. The antioxidant supplements significantly reduced DNA damage in cats with renal insufficiency as evidenced by reduced serum 8-OHdG and comet assay parameters. Therefore, supplements of vitamins E and C and beta-carotene as antioxidants may be beneficial to cats with renal disease.