The possible beneficial effect of regular exercise training on the progression of chronic renal failure was studied in a prospective randomized controlled study. Thirty patients with a median glomerular filtration rate (GFR) of 25 ml/(min.1.73 m2) (range 10-43) were randomized to physical training (30 min of bicycling daily or an equal amount of other physical activities) or to maintenance of the usual lifestyle. The median maximal work capacity increased significantly in the exercise group and remained unchanged in the control group during a median observation time of 20 months whereas the rate of progression judged by the slope of GFR versus time plot was equal in the two groups. Hence, the beneficial effect of exercise training, earlier observed in rat studies, could not be reproduced in our patients. Physical exercise had no untoward effect on progression of renal disease.