A total of 105 subjects with impaired glucose tolerance were classified into two groups, 51 subjects with plasma glucose > 11.1 mmol l-1 in one of the blood samplings during OGTT, but at 2 h being less than < 11.1 mmol l-1 were classified as early hyperglycaemics. Fifty-four cases were classified as true IGT, with fasting plasma glucose < 7.8 mmol l-1 and post plasma glucose level between 7.8 and 11.1 mmol l-1. Age and sex matched groups of normals (healthy adults) and NIDDM cases without symptomatic secondary complications were also included in the study. Lipid peroxidation (LPO) product in plasma, erythrocyte, and erythrocyte cell membrane were found to be significantly elevated (p < 0.001) in IGT, early hyperglycaemia and diabetes mellitus while glycosylated haemoglobin was also higher. Antioxidant enzymes superoxide dismutase and catalase were significantly lower in red blood cells obtained from IGT and early hyperglycaemic groups. They were closer to the levels showed in NIDDM confirming that antioxidant deficiency is already present in subjects classified as impaired glucose tolerant. Among the antioxidant scavengers, reduced glutathione (GSH) and ascorbic acid are reduced by 15% and 20% in IGT and NIDDM, respectively. We conclude that antioxidant status is poor in both IGT and NIDDM, suggesting an overlap of frank diabetic state in those classified as IGT. It is possible that antioxidant therapy might retard progression from IGT to NIDDM.