Context. Persuasive evidence has demonstrated that increased physical activity is an associated with substantial reduction in risk of coronary heart disease. However, the role of physical activity in the prevention of stroke is less well established. Objective. To examine the association between physical activity and risk of total stroke and stroke subtypes in women. Design and Setting. The Nurses' Health Study, a prospective cohort study of subjects residing in 11 US states. Subjects. A total of 72488 female nurses aged 40 to 65 years who did not have diagnosed cardiovascular disease or cancer at baseline in 1986 and who completed detailed physical activity questionnaires in 1986, 1988, and 1992. Main Outcome Measure. Incident stroke occurring between baseline and June 1, 1994, compared among quintiles of physical activity level as measured by metabolic equivalent tasks (METs) in hours per week. Results. During 8 years (560087 person-years) of follow-up, we documented 407 incident cases of stroke (258 ischemic strokes, 67 subarachnoid hemorrhages, 42 intracerebral hemorrhages, and 40 strokes of unknown type). In multivariate analyses controlling for age, body mass index, history of hypertension, and other covariates, increasing physical activity was strongly inversely associated with risk of total stroke. Relative risks (RRs) in the lowest to highest MET quintiles were 1.00, 0.98, 0.82, 0.74, and 0.66 (P for trend = .005). The inverse gradient was seen primarily for ischemic stroke (RRs across increasing MET quintiles, 1.00, 0.87, 0.83, 0.76, and 0.52; P for trend = .003). Physical activity was not significantly associated with subarachnoid hemorrhage or intracerebral hemorrhage. After multivariate adjustment, walking was associated with reduced risk of total stroke (RRs across increasing walking MET quintiles, 1.00, 0.76, 0.78, 0.70, and 0.66; P for trend = .01) and ischemic stroke (RRs across increasing walking MET quintiles, 1.00, 0.77, 0.75, 0.69, and 0.60; P for trend = .02) Brisk or striding walking pace was associated with lower risk of total and ischemic stroke compared with average or casual pace. Conclusion. These data indicate that physical activity, including moderate-intensity exercise such as walking, is associated with substantial reduction in risk of total and ischemic stroke in a dose-response manner.