OBJECTIVES: To study whether exercise is associated with the risk of venous thrombosis in elderly people. DESIGN: Observational study with a median follow-up of 11.6 years. SETTING: The Cardiovascular Health Study in four U.S. communities. PARTICIPANTS: People aged 65 and older without prior venous thrombosis (deep venous thrombosis or pulmonary embolism). MEASUREMENTS: Self-reported exercise was measured two or three times during follow-up and was defined as expending more than 500 kcal/wk on exercise, including walking for exercise. Venous thrombosis cases were verified using medical record review. RESULTS: Of 5,534 participants, 171 developed a first venous thrombosis. Self-reported exercise at baseline was not related to the risk of venous thrombosis after adjustment for sex, age, race, self-reported health, and body mass index (adjusted hazard ratio (HR(adj))=1.16, 95% confidence interval (CI)=0.84-1.61), although with exercise modeled as a time-varying exposure, overall results were in the direction of greater risk of venous thrombosis (HR(adj)=1.38, 95% CI=0.99-1.91). For mild-intensity exercise, such as walking, there was a nonsignificant finding in the direction of benefit (HR(adj)=0.75, 95% CI=0.49-1.16), but strenuous exercise, such as jogging, was associated with greater risk of venous thrombosis (HR(adj)=1.75, 95% CI=1.08-2.83) than no exercise at all. CONCLUSION: In elderly people, strenuous exercise was associated with a higher risk of venous thrombosis than no exercise at all. Future studies are needed to explain this unexpected higher risk.