The American Diabetes Association recently proposed a new, lower, cut-point of 7.0 mmol/l for diagnosis of diabetes mellitus. We examined data from the Caerphilly and Speedwell cohorts to determine possible cut-points of fasting plasma glucose for increased risk of subsequent ischaemic heart disease (IHD). Men (n = 4860) from the general population of a town in South Wales and a practice-based population in Bristol aged 45-63 years were first examined in 1979-83, and re-examined at intervals, and these data relate to follow-up at about 10 years (120 months, Caerphilly) (112 months, Speedwell). Clinically recognized diabetics (n = 94) experienced a higher mortality rate and an excess number of major IHD events. Among non-diabetics, mean blood glucose was 5.0 mmol/l and a significant excess of major IHD events occurred above this point even when the data were fully adjusted for all other IHD risk factors. Risk of major IHD was greatest for non-diabetic men with plasma glucose levels between 7.0 and 7.7 mmol/l. Under 7.0 mmol/l, the excess event rate was modest, however. Glucose levels were not associated with excess all-cause mortality among these non-diabetic men. These data, based on the excess risk of macrovascular disease experienced by a British cohort of non-diabetic men, accord with the proposals to base the diagnosis of diabetes on a cut point of 7.0 rather than 7.8 mmol/l.