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]