BACKGROUND: Age-related endothelial dysfunction and vascular stiffening are associated with increased cardiovascular (CV) risk. Many groups have encouraged goals of > 10,000 steps/day or > 30 min/day of moderate intensity physical activity (MPA) to reduce age-related CV risk. The impact of MPA on the vasculature of older adults remains unclear. METHODS AND RESULTS: We randomized 114 sedentary older adults ages > 50 to 12 weeks of either no intervention (group 1), a pedometer-only intervention (group 2), or a pedometer with an interactive website employing strategies to increase the adoption of habitual physical activity (PA, group 3). Endothelial function by brachial flow-mediated dilation (FMD%), vascular stiffness by tonometry, step-count by pedometer, and PA intensity/distribution by accelerometer were measured. Step-count increased in groups 2 (5,136 +/- 1,554 to 9,596 +/- 3,907, p < 0.001) and 3 (5,474 +/- 1,512 to 8,167 +/- 3,111, p < 0.001) but not in group 1 (4,931 +/- 1,667 to 5,410 +/- 2,410). Both groups 2 and 3 increased MPA > 30 min/day. Only group 3 increased MPA in continuous bouts of > 10 minutes (p < 0.001) and improved FMD% (p = 0.001). Neither achievement of > 10,000 steps/day nor > 30 min/day of MPA resulted in improved FMD%. However, achieving > 20 min/day in MPA bouts resulted in improved FMD%. No changes in vascular stiffness were observed. CONCLUSIONS: MPA reverses age-related endothelial dysfunction, but may require MPA to be performed in bouts of > 10 minutes duration for > 20 min/day to be effective. Commonly encouraged PA goals do not guarantee improved endothelial function and may not be as effective in reducing CV risk. CLINICAL TRIAL REGISTRATION: URL ClinicalTrials.gov. Unique identifier NCT01212978.

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