Hyperglycaemia leads to ROS (Reactive oxygen species) generation, affecting the cells that cannot decrease glucose uptake such as: glomerular epithelial cells, mesangial cells and proximal tubule cells. ROS excess seems to activate important pathogenic pathways of development of diabetic nephropathy. The decrease of CAT activity, one of the most important antioxidant enzymes, following to some genetic defects, may be a risk factor for diabetic nephropathy. The purpose of this study is to investigate the association of 21A/T (rs7943316) polymorphism of CAT gene with advanced diabetic nephropathy in patients with type 1 diabetes in Romania. There have been studied 238 patients with T1D (type 1 diabetes), divided into the group with diabetic nephropathy (DN) (106 patients) and the group without renal affectation (132 patients). The genotyping has been made by using PCR-RFLP technique. The analysis of association has been made by using DeFinetti programme. The value considered significant has been p < 0.05. There has been a deviation from Hardy-Weinberg equilibrium in the group with diabetic nephropathy (p = 0.019), the equilibrium being preserved by the control group (p = 0.771). T allele does not confer a risk for advanced diabetic nephropathy (ORT = 0.757, 95% C.I. = 0.405-1.414; P = 0.381), the result being statistically insignificant even taking into consideration the risk allele A (ORA = 0.793, 95% C.I. = 0.465-1.350; P = 0.392). The results remain concordant too after applying the Cochran -Armitage test. Our data do not suggest an effect of 21A/T (rs7943316) polymorphism in the susceptibility for diabetic nephropathy in Romanian patients with type 1 diabetes. Further studies are necessary in order to demonstrate or exclude the role of CAT gene in diabetic nephropathy in patients with type 1 diabetes.