Impaired glucose tolerance (IGT) has been investigated in a large number of populations from different parts of the world and with differing lifestyles. The occurrence of IGT varies widely. IGT prevalence tends to increase with age, but there is no consistent relationship with gender. Prevalence is lower in groups with less obesity and having physically active lifestyles, such as those living in rural areas and having traditional lifestyles, even when subjects with the same genetic background are compared. Risk factors for IGT are similar to those for non-insulin dependent diabetes mellitus (NIDDM). IGT, characterized by hyperglycaemia and insulin resistance, is probably a stage in the pathogenesis of NIDDM. Therapies aimed at improving glucose tolerance and decreasing insulin resistance in subjects with IGT may, therefore, be able to delay or prevent the development of NIDDM.