The benefit of weight reduction for cardiovascular disease (CVD) outcomes remains uncertain. To examine the effects of baseline body mass index on major CVD outcomes and diabetes over a 20 year follow up, and of weight change in the first five years over the subsequent 15 years. A prospective study of British men followed up for 20 years. Men aged 40-59 years with no diagnosis of CVD or diabetes (n = 7176) of whom 6798 provided full information on weight change five years later. Major CVD events (fatal and non-fatal myocardial infarction and stroke, angina, "other" CVD deaths) and diabetes. During the 20 year follow up there were 1989 major CVD events and 449 incident cases of diabetes in the 7176 men. Risk of major CVD and diabetes increased significantly with increasing overweight and obesity. During the 15 year follow up, weight gain was associated with increased risk of CVD and diabetes. Weight loss was associated with lower risk of diabetes than the stable group irrespective of initial weight. No significant cardiovascular benefit was seen for weight loss in any men, except possibly in considerably overweight (BMI 27.5-29.9 kg/m(2)) younger middle aged men (RR = 0.42; 95% CI 0.22 to 0.81). Long term risk of CVD and diabetes increased significantly with increasing overweight and obesity. Weight loss was associated with significant reduction in risk of diabetes but not CVD, except possibly in considerably overweight younger men. Duration and severity of obesity seem to limit the cardiovascular benefits of weight reduction in older men.