The impact of cigarette smoking on stroke incidence was assessed in the Framingham Heart Study cohort of 4255 men and women who were aged 36 to 68 years and free of stroke and transient ischemic attacks. During 26 years of follow-up, 459 strokes occurred. Regardless of smoking status and in each sex, hypertensive subjects had twice the incidence of stroke. Using the Cox proportional hazard regression method, smoking was significantly related to stroke after age and hypertension were taken into account. Even after pertinent cardiovascular disease risk factors were added to the Cox model, cigarette smoking continued to make a significant independent contribution to the risk of stroke generally and brain infarction specifically. The risk of stroke increased as the number of cigarettes smoked increased. The relative risk of stroke in heavy smokers (greater than 40 cigarettes per day) was twice that of light smokers (fewer than ten cigarettes per day). Lapsed smokers developed stroke at the same level as nonsmokers soon after stopping. Stroke risk decreased significantly by two years and was at the level of nonsmokers by five years after cessation of cigarette smoking.