© 2017 by the American Society of Nephrology. CKD appears to be a condition of soluble klotho deficiency. Despite known associations between low soluble klotho levels and conditions that promote kidney damage, such as oxidative stress and fibrosis, little information exists regarding the longitudinal association between soluble klotho levels and change in kidney function. We assayed serum soluble a-klotho in 2496 participants within the Health Aging and Body Composition study, a cohort of older adults. The associations between soluble klotho levels and decline in kidney function (relative decline: EGFR decline $30%; absolute decline: EGFR decline .3 ml/min per year) and incident CKD (incident EGFR ,60 ml/min per 1.73 m2 and .1 ml/min per year decline) were evaluated. We adjusted models for demographics, baseline EGFR, urine albumin-To-creatinine ratio, comorbidity, and measures of mineral metabolism. Among participants, the mean (SD) age was 75 (3) years, 52% were women, and 38% were black. Median (25th, 75th percentiles) klotho level was 630 (477, 817) pg/ml. In fully adjusted models, each two-fold higher level of klotho associated with lower odds of decline in kidney function (odds ratio, 0.78 [95% confidence interval, 0.66 to 0.93] for 30%decline in EGFR, and 0.85 [95% confidence interval, 0.73 to 0.98] for.3ml/min per year decline in EGFR), but not of incident CKD (incident rate ratio, 0.90 [95% confidence interval, 0.78 to 1.04]). Overall, a higher soluble klotho level independently associated with a lower risk of decline in kidney function. Future studies should attempt to replicate these results in other cohorts and evaluate the underlying mechanism.