The aim of the present study was to investigate whether hyperoxic aerobic high intensity one-legged interval cycling improves peak oxygen uptake (V O (2peak)) compared to normoxic training in patients with severe chronic obstructive pulmonary disease, and to evaluate the acute effect of hyperoxia during one- and two-legged peak exercise testing. Twelve COPD patients were recruited to perform 3 training sessions per week for 8 weeks in hyperoxia (n=7) or normoxia (n=5). Each leg was trained 4x4 min at 85-95% of the one-legged peak heart rate. One-legged V O (2peak) increased in the hyperoxia and normoxia training groups by 24 and 15% (16.1(13.2)-20.0(11.3) and 17.4(15.1)-20.0(6.7) mL.kg (-1).min (-1)) respectively. The corresponding increases in V O (2peak) during two-legged testing were 14% in both groups (20.1(11.5)-22.9(10.6) and 18.8(8.5)-21.4(7.3) mL.kg (-1).min (-1)). There were no differences between groups from pre- to post-training. Nor were there any differences between acute hyperoxia and normoxia at the pre- or post-peak exercise test. One-legged aerobic high intensity interval cycling significantly increases V O (2peak) in COPD patients. However, breathing supplemental oxygen during training or testing does not appear to improve V O (2peak) above the level attained by breathing ambient air.