Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) has been reported to serve as a sensitive biomarker of oxidative DNA damage and also of oxidative stress. We have investigated oxidative DNA damage in patients with non-insulin-dependent diabetes mellitus (NIDDM) by urinary 8-OHdG assessments. We determined the total urinary excretion of 8-OHdG from 24 h urine samples of 81 NIDDM patients 9 years after the initial diagnosis and of 100 non-diabetic control subjects matched for age and gender. The total 24 h urinary excretion of 8-OHdG was markedly higher in NIDDM patients than in control subjects (68.2 +/- 39.4 microg vs. 49.6 +/- 37.7 microg, P = 0.001). High glycosylated hemoglobin was associated with a high level of urinary 8-OHdG. The increased excretion of urinary 8-OHdG is seen as indicating an increased systemic level of oxidative DNA damage in NIDDM patients.