The diabetic patient is at significantly increased risk of developing vascular disease. Its aetiology may involve oxidative damage by free radicals and protection against such damage can be offered by radical-scavenging antioxidants. We investigated whether there was a relationship between glycaemic control as assessed by measurement of glycated haemoglobin (HbA1c) and serum antioxidant status in a population of 118 diabetic outpatients with either insulin-dependent or non-insulin-dependent diabetes. Amongst patients with non-insulin-dependent diabetes mellitus there was a significant inverse correlation between levels of glycated haemoglobin and total free radical scavenging activity (r = -0.456, P < 0.0001). This association resulted primarily because of a similar correlation with uric acid (r = -0.421, P = 0.0003). There was also a weak inverse correlation with vitamin A but no significant association with vitamin C or vitamin E levels. There were no significant associations found amongst the patients with insulin-dependent diabetes. These results indicate that poor diabetic control is associated with reduced serum free radical scavenging (antioxidant) activity in non-insulin-dependent diabetes mellitus. By implication improved glycaemic control may preserve serum antioxidant status in diabetes.