Aims/hypothesis. The increased oxidative stress in diabetes is known to contribute to the progression of diabetes and its complications. We have reported a significant relation between the content of 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG), a product of oxidative DNA damage in urine or leukocytes and the severity of diabetic nephropathy and retinopathy [1]. We investigated whether 8-oxodG in urine or leukocytes is associated with the progression of diabetic nephropathy. Methods. We measured urinary 8-oxodG contents at entry and carried out a prospective longitudinal study to assess the progression of nephropathy over 5 years.Results. There was a significant progression of diabetic nephropathy in the patients with higher excretion of 8-oxodG in urine compared with the patients with moderate or lower excretion of 8-oxodG. There was no significant association between the leukocyte 8-oxodG contents and the development of nephropathy. The multivariate logistic regression analysis suggests that the urinary 8-oxodG was the strongest predictor of nephropathy among several known risk factors. Conclusion/interpretation. This study provides evidence that increased oxidative stress has a primary role in the pathogenesis of diabetic nephropathy. A local enhancement of oxidative stress in diabetic kidney might explain the possible linkage between the increased urinary excretion of 8-oxodG and the development of nephropathy. 8-oxodG in urine is a useful clinical marker to predict the development of diabetic nephropathy in diabetic patients. Oxidative stress is one of the important mediators of vascular complications in diabetes including nephropathy. The hyperglycaemia-induced generation of reactive oxygen species (ROS) is linked by three pathological pathways to diabetic complications: activation of protein kinase C (PKC), the formation of advanced glycation end products (AGE), and sorbitol accumulation [4,5]. Normalising mitochondrial ROS production was reported to block these hyperglycaemia-mediated signalling pathways.8-oxodG is a product of oxidative DNA damage following specific enzymatic cleavage after ROSinduced 8-hydroxylation of the guanine base in mitochondrial and nuclear DNA [6]. 8-oxodG is known to be a sensitive marker of oxidative DNA damage and of the total systemic oxidative stress in vivo [7]. SevThe link between hyperglycaemia and the complications of diabetes is still not clear. Increased oxidative stress in diabetes might contribute to the pathogenesis of diabetic complications, including nephropathy [2,