Impaired glucose tolerance (IGT) is associated with increased mortality due to ischaemic heart disease (IHD), but as it is not known whether this excess mortality can be reduced by preventing or delaying the development of non-insulin-dependent diabetes mellitus (NIDDM), a long-term NIDDM prevention trial of dietary counselling and physical exercise was launched at Malmö, Sweden, the 12-year follow-up of which is reported here. At 12-year follow-up of 6956 men who underwent health screening at 48 years of age, an IGT intervention group (n = 288) who participated in a long-term NIDDM prevention programme were compared with an IGT non-randomised routine treatment group (n = 135), a diabetic group (n = 144), and the remainder, the normal glucose tolerance (NGT) group (n = 6389). The variables studied included the levels of blood glucose, plasma insulin, blood pressure, blood lipids, lung function and maximum oxygen uptake. Subjects with IGT were characterised by overweight, poor vital capacity, hypertension, hypertriglyceridaemia and hyperinsulinaemia. The mortality rate in the IGT intervention group was similar to that in the NGT group (6.5 vs 6.2 per 1000 person years at risk) and lower than that in the IGT routine treatment group (6.5 vs 14.0, p = 0.009). In the two IGT groups taken together, intervention but not body mass index, systolic blood pressure, smoking, cholesterol or the 2-h glucose level predicted mortality. Systolic blood pressure was a predictor of IHD mortality among IGT subjects; and in the cohort as a whole, body mass index, systolic blood pressure, hypercholesterolaemica, diabetes and smoking were predictors of IHD mortality. The findings suggest that a long-term intervention programme, with an emphasis on lifestyle changes, including dietary counselling and physical exercise, will reduce mortality in subjects with IGT who are at an increased risk of both developing NIDDM and of premature death due to IHD and other causes.