The WHO and the National Diabetes Data Group have produced definitions to classify subjects with blood glucose levels following oral ingestion that are higher than normal, but are less than for patients with diabetes. The term impaired glucose tolerance (IGT) has been used to describe this group who have an increased risk of developing non-insulin dependent diabetes mellitus and cardiovascular disease, compared with subjects with normal glucose tolerance. Wide intra-subject variability has been observed when glucose tolerance tests are carried out on separate occasions. This has led some researchers to suggest that IGT merely constitutes a blurred boundary between normality and diabetes but accumulating evidence now supports the hypothesis that IGT is a useful risk factor category.