OBJECTIVE--To determine the risk factors for noninsulin dependent diabetes in a cohort representative of middle aged British men. DESIGN--Prospective study. SUBJECTS AND SETTINGS--7735 men aged 40-59, drawn from one group practice in each of 24 towns in Britain. Known and probable cases of diabetes at screening (n = 158) were excluded. MAIN OUTCOME MEASURES--Non-insulin dependent diabetes (doctor diagnosed) over a mean follow up period of 12.8 years. RESULTS--There were 194 new cases of non-insulin dependent diabetes. Body mass index was the dominant risk factor for diabetes, with an age adjusted relative risk (upper fifth to lower fifth) of 11.6; 95% confidence interval 5.4 to 16.8. Men engaged in moderate levels of physical activity had a substantially reduced risk of diabetes, relative to the physically inactive men, after adjustment for age and body mass index (0.4; 0.2 to 0.7), an association which persisted in full multivariate analysis. A nonlinear relation between alcohol intake and diabetes was observed, with the lowest risk among moderate drinkers (16-42 units/week) relative to the baseline group of occasional drinkers (0.6; 0.4 to 1.0). Additional significant predictors of diabetes in multivariate analysis included serum triglyceride concentration, high density lipoprotein cholesterol concentration (inverse association), heart rate, uric acid concentration, and prevalent coronary heart disease. CONCLUSION--These findings emphasise the interrelations between risk factors for non-insulin dependent diabetes and coronary heart disease and the potential value of an integrated approach to the prevention of these conditions based on the prevention of obesity and the promotion of physical activity.