Hyperglycaemia, a key clinical manifestation of diabetes mellitus, not only generates reactive oxygen species (ROS), but also attenuates anti-oxidative mechanisms by scavenging enzymes and antioxidant substances [1]. As ROS cause strand breaks in DNA and base modifications including the oxidation of guanine residues to 8-oxo, 2 ¢-deoxyguanosine (8-oxodG), 8-oxodG can serve as a sensitive biomarker of oxidative DNA damage [2]. 8-OxodG was increased in the kidneys of diabetic rats, and insulin treatment reduced both urinary albumin excretion and 8-oxodG formation in the kidney [3]. A recent study reported an increase in the 8-oxodG content in mononuclear cells and ROS level in Type I (insulin-dependent) and Type II (non-insulin-dependent) diabetic patients when compared with control subjects [4]. Another study reported that urinary 8-oxodG excretion was higher in Type II diabetic patients than in the control subjects [5]. Urinary 8-oxodG excretion correlated with glycated haemoglobin [5]. We speculated that diabetes-associated modifications of DNA by ROS might contribute to the diabetic complications. Diabetologia (1999) Abstract Aims/hypothesis. Augmented oxidative stress induced by hyperglycaemia possibly contributes to the pathogenesis of diabetic complications. Oxidative stress is known to increase the conversion of deoxyguanosine to 8-oxo, 2 ¢-deoxyguanosine in DNA. To investigate the possible contribution of oxidative DNA damage to the pathogenesis of diabetic complications, we measured the content of 8-oxo, 2 ¢-deoxyguanosine in the urine and the blood mononuclear cells of Type II (non-insulin-dependent) diabetic patients. Methods. We studied 53 Type II diabetic patients and 39 age-matched healthy control subjects. We assayed 8-oxo, 2 ¢-deoxyguanosine by HPLC-electrochemical detection method. Results. The content of 8-oxo, 2 ¢-deoxyguanosine in the urine and the mononuclear cells of the Type II diabetic patients was much higher than that of the control subjects. Urinary 8-oxo, 2 ¢-deoxyguanosine excretion and the 8-oxo, 2 ¢-deoxyguanosine content in the mononuclear cells from the diabetic patients with complications were higher than those from the diabetic patients without complications. Urinary excretion of 8-oxo, 2 ¢-deoxyguanosine was significantly correlated with the 8-oxo, 2 ¢-deoxyguanosine content in the mononuclear cells. The 8-oxo, 2 ¢-deoxyguanosine content in the urine and mononuclear cells was correlated with the haemoglobin A 1 c value. Conclusion/interpretation. This is the first report of a direct association between oxidative DNA damage and the complications of diabetes. The augmented oxidative DNA damage in diabetes is speculated to contribute to the pathogenesis of diabetic complications. [Diabetologia (1999) 42: 995±998]