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BACKGROUND Greater hippocampal volume is observed in healthy older adults after short-term structured exercise. Whether long-term exposure to real-world physical activity (PA) programs has similar effects for sedentary older adults with impaired mobility and comorbid conditions is not known. HYPOTHESIS A long-term moderate intensity regimen of PA is related to larger volume of the hippocampus in older adults at risk for mobility disability. We further explore whether these associations are modified by factors known to be related to dementia. METHODS Twenty-six sedentary adults at risk for mobility disability participated in a 24-month randomized intervention program of physical activity (PA, n=10, age: 74.9 years, 7 women) or health education (HE, n=16, age: 76.8 years, 14 women). Volumes of total hippocampus, dentate gyrus and cornu ammonis were measured at baseline and at 24-month follow-up using 7 Tesla MRI. Between-group volumetric differences at 24-months were adjusted for sessions attended and baseline volumes. The contribution of each dementia-related factor was tested separately for education, APOE, diabetes, cardiovascular diseases, white matter hyperintensities and brain atrophy. RESULTS Between-group differences were significant for left hippocampus, left cornu ammonis and right hippocampus. Adjustment for regional baseline volume attenuated the associations to statistically non-significant for right hippocampus and left conru ammonis; associations for left hippocampus were robust for all adjustments. Results were similar after adjustment for dementia-related factors. CONCLUSIONS In this group of sedentary older adults there was a hippocampal response to a long-term program of moderate-intensity PA. Future studies should examine whether hippocampal response could explain the beneficial effects of PA on cognition for vulnerable older adults.