Context: Physical activity has been related to reduced mortality, but it is not clear whether changes in physical activity affect mortality among older women. Objective: To examine the relationship of changes in physical activity and mortality among older women. Design, Setting, and Participants: Prospective cohort study conducted at 4 US research centers (Baltimore, Md; Portland, Ore; Minneapolis, Minn; and Monongahela Valley, Pa) among 9518 community-dwelling white women aged 65 years or older who were assessed at baseline (1986-1988), 7553 of whom were reassessed at a follow-up visit (1992-1994; median, 5.7 years later). Main Outcome Measures: Walking and other physical activities at baseline and follow-up; vital status, with cause of death confirmed by death certificates/discharge summaries, tracked for up to 12.5 years after baseline (up to 6.7 years after the follow-up visit). Results: Compared with continually sedentary women, those who increased physical activity levels between baseline and follow-up had lower mortality from all causes (hazard rate ratio [HRR], 0.52; 95% confidence interval [CI], 0.40-0.69), cardiovascular disease (HRR, 0.64; 95% CI, 0.42-0.97), and cancer (HRR, 0.49; 95% CI, 0.29-0.84), independent of age, smoking, body mass index, comorbid conditions, and baseline physical activity level. Associations between changes in physical activity and reduced mortality were similar in women with and without chronic diseases but tended to be weaker among women aged at least 75 years and among those with poor health status. Women who were physically active at both visits also had lower all-cause mortality (HRR, 0.68; 95% CI, 0.56-0.82) and cardiovascular mortality (HRR, 0.62; 95% CI, 0.44-0.88) than sedentary women. Conclusion: Increasing and maintaining physical activity levels could lengthen life for older women but appears to provide less benefit for women aged at least 75 years and those with poor health status.