The cardiorespiratory and metabolic response to exercise was evaluated in children with end-stage renal failure maintained on haemodialysis. Eight patients (haemodialysis group), 4 boys and 4 girls, with a mean age of 13.4 +/- 3.6 years (range 9.4-18.6 years) and haemoglobin levels ranging from 5.3 to 7.6 g/dl and 16 healthy children (control group) performed a progressive exercise testing on a treadmill. Gas exchange was simultaneously monitored. The mean ventilatory anaerobic threshold of the haemodialysis group, expressed as a percentage of the reference values, was 59.1 +/- 18.2%, and their maximum work load (29.9 +/- 19 W) was about one fourth of that reached by the control group (113.3 +/- 51.6 W). Ventilatory anaerobic threshold values in the haemodialysis group significantly correlated with blood haemoglobin levels, but not with creatinine and parathyroid hormone concentrations. We, therefore, conclude (1) that children maintained on chronic haemodialysis have a marked reduction in aerobic working capacity and (2) that the major cause for this limitation appears to be the reduced haemoglobin concentration.