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Introduction: Leisure time physical activity (LTPA) has been consistently associated with a lower risk of stroke across several populations. Less is known however about how changes over the life-time in physical activity may influence the risk of stroke. We examined the association of changes in engaging in moderate and strenuous physical activity with risk of stroke in the California Teachers Study.Methods: The California Teachers Study is a prospective cohort study established in 1995. Baseline LTPA was obtained using a questionnaire outlining time spent in moderate (brisk walking for example) and strenuous (swimming and running for example); the same questionnaire was repeated in 2005-6. We defined the exposure as engaging in any activity for at least 15 minutes per week (any) in the second questionnaire when not engaging in any on the first questionnaire, with the reference being no change or a decline in activity between both. Multi-variable Cox models were constructed to examine the association of the change in LTPA with risk of stroke. Separate models were created for moderate and strenuous activity.Results: There were a total of 61,256 participants with two LTPA questionnaires available separated by 10 years. A total of 3,111 participants increased their level of moderate activity, 11,744 remained active in both, 2,760 declined from the first to the second questionnaire, while 2,508 reported no activity in both. There were 987 (709 ischemic, 221 hemorrhagic, 247 deaths) strokes after the second questionnaires (mean follow up 6.5 years). In multi-variable models increasing any moderate intensity from none from the two questionnaires, versus no change or decline in activity, was associated with a lower risk of all stroke (adjusted HR 0.83, 95% CI 0.72-0.95). We found no associations of changes in strenuous LTPA with risk of stroke (adjusted HR 0.99, 95% CI 0.83-1.17). The associations were statistically significant for ischemic stroke (adjusted HR 0.75, 95% CI 0.64-0.89) but not hemorrhagic stroke (adjusted HR 0.94, 95% CI 0.72-1.25).Conclusion: Improving or maintaining moderate intensity physical activity is associated with a lower risk of stroke, while changes in strenuous activity is not.