Backgrounds and aims: Paraoxonase 1 (PON1) is a novel marker that has been shown to exert protective functions on atherosclerosis by preventing oxidative modification of serum lipoproteins. In this study, we investigated the effects of PON1 on CA-IMT in renal transplant patients. Methods: A total of 98 adult renal transplant recipients was enrolled in the study. CA-IMT was determined by B-mode Doppler ultrasonography. PON1 activity was assessed by the rate of enzymatic hydrolysis of paraoxon to p-nitrophenol. Results: Mean age was 39.4 +/- 9.6 years and 10% of the patients were diabetic. Time after transplant was 76 +/- 59 months. Mean PON1 level was 62.1 +/- 43.3 U/l. PON1 levels were negatively correlated with CA-IMT and positively with HDL-cholesterol. Mean CA-IMT was 0.62 +/- 0.10 mm (0.40 - 0.98). CA-IMT was positively correlated with age, male gender and negatively with proteinuria and PON1 levels. In linear regression analysis, PON1 levels were associated with CA-IMT. Conclusion: Reduced PON1 activity is significantly associated with increased carotid atherosclerosis in renal transplant patients.