Background: Limited evidence suggests that physical activity may prevent frailty and associated negative outcomes in older adults. Definitive data from large long-term randomized trials are lacking. Objective: To determine whether a long-term, structured, moderate-intensity physical activity program is associated with a lower risk for frailty and whether frailty status alters the effect of physical activity on the reduction in major mobility disability (MMD) risk. Design: Multicenter, single-blind, randomized trial. Setting: 8 centers in the United States. Participants: 1635 community-dwelling adults, aged 70 to 89 years, with functional limitations. Intervention: A structured, moderate-intensity physical activity program incorporating aerobic, resistance, and flexibility activities or a health education program consisting of workshops and stretching exercises. Measurements: Frailty, as defined by the SOF (Study of Osteoporotic Fractures) index, at baseline and 6, 12, and 24 months, and MMD, defined as the inability to walk 400 m, for up to 3.5 years. Results: Over 24 months of follow-up, the risk for frailty (n = 1623) was not statistically significantly different in the physical activity versus the health education group (adjusted prevalence difference, -0.021 [95% CI, -0.049 to 0.007]). Among the 3 criteria of the SOF index, the physical activity intervention was associated with improvement in the inability to rise from a chair (adjusted prevalence difference, -0.050 [CI, -0.081 to -0.020]). Baseline frailty status did not modify the effect of physical activity on reducing incident MMD (P for interaction = 0.91). Limitation: Frailty status was neither an entry criterion nor a randomization stratum. Conclusion: A structured, moderate-intensity physical activity program was not associated with a reduced risk for frailty over 2 years among sedentary, community-dwelling older adults. The beneficial effect of physical activity on the incidence of MMD did not differ between frail and nonfrail participants.