OBJECTIVES: To examine associations between weight change, body composition, risk of mobility disability, and mortality in older adults. DESIGN: Prospective, longitudinal, population-based cohort. SETTING: The Health, Aging, and Body Composition Study. PARTICIPANTS: Women (n = 1,044) and men (n = 931) aged 70 to 79. MEASUREMENTS: Weight and lean and fat mass from dual-energy X-ray absorptiometry measured annually over 5 years. Weight was defined as stable (n = 664, reference), loss (n = 662), gain (n = 321), or cycling (gain and loss, n = 328) using change of 5% from year to year or from Year 1 to 6. Mobility disability (two consecutive reports of difficulty walking one-quarter mile or climbing 10 steps) and mortality were determined for 8 years after the weight change period. Associations were analyzed using Cox proportional hazards regression adjusted for covariates. RESULTS: During follow-up, 313 women and 375 men developed mobility disability, and 322 women and 378 men died. There was no risk of mobility disability or mortality with weight gain. Weight loss (hazard ratio (HR) = 1.88, 95% confidence interval (CI) = 1.40-2.53) and weight cycling (HR = 1.59, 95% CI = 1.11-2.29) were associated with mobility disability in women, and weight loss was associated with mobility disability in men (HR = 1.30, 95% CI = 1.01-1.69). Weight loss and weight cycling were associated with mortality risk in women (weight loss: HR = 1.47, 95% CI = 1.07-2.01; weight cycling: HR = 1.62, 95% CI = 1.15-2.30) and in men (weight loss: HR = 1.41, 95% CI = 1.09-1.83; weight cycling: HR = 1.50, 95% CI = 1.08-2.08). Adjustment for lean and fat mass and change in lean and fat mass from Year 1 to 6 attenuated the relationships between weight loss and mobility disability in men and between weight loss and mortality in men and women. CONCLUSION: Weight cycling and weight loss predict impending mobility disability and mortality in old age, underscoring the prognostic importance of weight history.