Physical activity may counteract metabolic disturbances that promote the progression of CKD. To address this concept, we performed a longitudinal cohort study of 256 participants in the Seattle Kidney Study, a clinic-based study of CKD. Participants with an estimated GFR (eGFR) of 15-59 ml/min per 1.73 m(2) at baseline were eligible for the study. Physical activity was quantified using the Four-Week Physical Activity History Questionnaire. We used generalized estimating equations to test associations of physical activity with change in eGFR determined by longitudinal measurements of serum cystatin C. Mean baseline eGFR was 42 ml/min per 1.73 m(2). During a median 3.7 years of follow-up, the mean change in eGFRcystatin C was -7.6% per year (interquartile range, -16.8%, 4.9% per year). Participants who reported >150 minutes of physical activity per week had the lowest rate of eGFRcystatin C loss (mean -6.2% per year compared with -9.6% per year among inactive participants). In adjusted analyses, each 60-minute increment in weekly physical activity duration associated with a 0.5% slower decline per year in eGFR (95% confidence interval, 0.02 to 0.98; P=0.04). Results were similar in sensitivity analyses restricted to participants without cardiovascular disease or diabetes, or to participants with moderate/high physical function. After adjustment for eGFR at the time of questionnaire completion, physical activity did not associate with the incidence of ESRD (n=34 events). In summary, higher physical activity levels associated with slower rates of eGFR loss in persons with established CKD.