Impaired glucose tolerance (IGT) was introduced in 1979 as an intermediate category covering the grey area between unequivocal diabetes mellitus and risk free more normal glucose tolerance. The IGT group included those at high risk of subsequent development of non-insulin-dependent diabetes mellitus (NIDDM) but low risk of specific diabetic complications. Categorisation of subjects as IGT is hampered by the variability of the oral glucose tolerance test, but even those shown to be IGT only once are at increased risk of developing NIDDM. The relative roles of inheritance, fetal undernutrition, and environmental life style factors (physical inactivity and diet) in the aetiology and pathogenesis of IGT are discussed, with all contributing. The prevalence of IGT in different populations has now been widely studied with values ranging from 2 to 25% in adults. Rates of progression to NIDDM also vary widely from 2 to 14% per year. Risk factors for progression are discussed. IGT also carries an increased risk of development of cardiovascular disease (CVD) and forms part of the "metabolic syndrome". The role of insulin resistance as a common aetiological factor is briefly reviewed. Finally, possible means of treatment of IGT are listed with the intent of delaying the onset of diabetes and CVD, which is of obvious clinical importance.