Objective changes in exercise tolerance were evaluated in five chronic hemodialysis patients (HD) following a ten week aerobic treadmill exercise conditioning program designed to exercise patients at a level approximating their anaerobic threshold (AT). This exercise conditioning program resulted in a 21% increase in peak oxygen consumption, an increase in test duration from 17.40 +/- 1.26 to 22.60 +/- 0.87 min (P less than 0.025), and an increase in AT from 0.81 +/- 0.08 to 0.97 +/- 0.08 1/min (P less than 0.05). These physiologic changes in exercise tolerance resulted in an overall increase in work capacity permitting the HD patient to perform a larger portion of the imposed work load aerobically.