['To assess the extent to which physical activity and leg strength are associated with change in mobility in older persons.', 'Prospective, observational cohort study.', 'Retirement communities across the Chicago metropolitan area participating in the Rush Memory and Aging Project.', 'Eight hundred eighty-six ambulatory older persons without dementia.', 'Rate of change in mobility.', 'In a linear mixed-effects model that controlled for age, sex, education, and a term for baseline physical activity, a higher level of physical activity was associated with a slower rate of mobility decline (estimate=0.006, standard error (SE)=0.003, P=.03); each additional hour of physical activity at baseline was associated with an approximately 3% decrease in the rate of mobility decline. In a similar model, a higher level of baseline leg strength was associated with a slower rate of mobility decline (estimate=0.031, SE=0.132, P=.02); each additional unit of leg strength at baseline was associated with an approximately 20% decrease in the rate of mobility decline. In a final model, which included terms for physical activity and leg strength together, both were associated with decline in mobility. Furthermore, both remained associated with mobility even after controlling for body composition, balance, pulmonary function, cognition, history of joint pain, cardiovascular diseases and risk factors, and medications.'] Do physical activity and leg strength predict decline in mobility performance in older persons?