Impaired antioxidant defence is implicated in the development of cardiovascular complications in non-insulin-dependent diabetes (NIDDM). However, as many of these patients are elderly, observed changes in antioxidant status may be due to the patient's age rather than their disease. We sampled blood from 47 elderly NIDDM patients (21 male and 26 female; mean age +/- SD, 75.62 +/- 7.97 years), 66 young (30 male and 36 female; 24.52 +/- 4.72 years) and 58 healthy elderly volunteers (17 male and 41 female; 70.74 +/- 4.85 years), and measured the antioxidant glutathione, the marker for free-radical-damage lipid hydroperoxide products (LHP), vitamin E and total antioxidant capacity (TAC). There was a significant increase in LHP in the healthy elderly group compared with the young volunteers (3.14 +/- 1.5 vs. 2.14 +/- 1.38 mumol/l, p < 0.01). The values were much higher in NIDDM patients (7.02 +/- 2.29 mumol/l, p < 0.0001 vs. healthy elderly). There was a reduction in TAC in healthy elderly compared with the young (359.99 +/- 54.82 vs. 471.47 +/- 94.29 mumol/l trolox equivalents, p < 0.0001), but there was no further reduction in NIDDM patients. Similarly, glutathione was reduced to the same degree in healthy elderly and NIDDM patients (0.29 +/- 0.09, 0.30 +/- 0.11 vs. 0.54 +/- 0.19 mumol/l in young volunteers, p < 0.0001). Vitamin E concentrations were comparable in all groups (26.34 +/- 5.39 young volunteers, 31.50 +/- 8.23 healthy elderly and 30.98 +/- 9.03 mumol/l NIDDM patients), but after correction for serum cholesterol there was a significant reduction in the diabetic group compared with the young, but not with the elderly (5.54 +/- 1.55 vs. 6.67 +/- 1.86 vs. 6.31 +/- 1.85 (mumol/l)/(mmol/l), p < 0.01). We have demonstrated an age-dependent reduction in total antioxidant capacity and glutathione defence and an age-independent increase in LHP in elderly patients with NIDDM. Reduced concentrations of vitamin E were demonstrated in NIDDM patients compared with young, but not elderly, volunteers. Increased oxidative damage occurs independently of age in NIDDM patients despite comparable antioxidant defences in this age group.