Patients with cystic fibrosis (CF) have been shown to exhibit impaired oxygen uptake (V˙O2) kinetics independent of their physical fitness. This study investigated whether oxygen supplementation improves V˙O2 kinetics in CF as determined by cycle ergometry at submaximal exercise intensities using a pseudo-random binary sequence exercise test i.e. a simultaneous application of different frequencies of sinusoidal work. The subjects were 9 CF patients and 13 healthy controls (HC) and they exercised while breathing humidified and heated air with a fractional concentration of oxygen in inspired air (F
IO2) of either 0.21 or 0.40. With a F
IO2 of 0.21 the respiratory exchange ratio (R) was higher in CF than in HC both at rest (0.91 vs 0.81) and during exercise (0.97 vs 0.89). Oxygen saturation (SO2) was slightly lower in CF, but remained above 90% during exercise (92.7% vs 95.2%). Spectrum analysis revealed that in CF, the amplitude ratio (AR) between V˙O2 and exercise intensity was lower over a wide frequency range (P < 0.05). In addition, CF showed a larger negative phase shift (PS) at lower frequencies (P < 0.005). With a F
IO2 of 0.40, SO2 increased to about 97% in both groups; while R remained higher in CF (0.92) compared to HC (0.81). In the control group, the O2 supplement raised AR but the V˙O2 kinetics of the CF patients were not significantly affected. In HC the enhanced AR during oxygen supplementation would suggest a cardiopulmonary limitation of V˙O2 at the onset of submaximal exercise. In CF patients low AR and PS would indicate an attenuated V˙O2 response attributable to an impaired oxygen utilization in the muscles because the oxygen supplement normalised SO2 but failed to improve R and V˙O2 kinetics.