STUDY OBJECTIVE--The aim was to examine the risk of increasing overweight for death from stroke. DESIGN--This was a prospective cohort study, in which the main outcome measure was the mortality ratio for stroke with increasing body mass index. SETTING--Civil service departments, Whitehall, London. SUBJECTS--Participants were 17,753 men aged 40 to 64 years. MEASUREMENTS AND MAIN RESULTS--208 stroke deaths were recorded. Men aged 40 to 54 in the most overweight quintile of body mass index had a mortality ratio of 2.01 (95% confidence interval 0.9 to 4.7) compared to the thinnest quintile. The mortality ratio was 1.19 (95% CI 0.7 to 2.0) in men aged 55 to 64. The increase in risk was more apparent in non-smokers: age adjusted mortality ratio 2.58 (95% CI 1.2 to 5.7). When smoking status and overweight were considered in combination a gradient of the age adjusted mortality ratio was observed, from 1.0 in thinner/non-smokers up to 3.15 in fatter/current smokers. On the assumption that smoking and obesity cause strokes, an estimated 60% of strokes could be prevented if these two easily identifiable risk factors could be avoided. CONCLUSIONS--The risks of overweight for death from stroke were more apparent in younger subjects and non-smokers. A substantial proportion of stroke deaths occurring under the age of 80 years would probably be prevented if cigarette smoking and overweight could be avoided.