Background and objective: Hyperoxia has been shown to reduce resting ventilation, hyperinflation and dyspnoea in patients with severely hypoxaemic COPD. This study assessed the effects of hyperoxia on these resting measures in patients with COPD of varying disease severity and characterized those patients who responded. Methods: Measurements of dyspnoea (Borg score), oxyhaemoglobin saturation (SpO2), inspiratory capacity (IC), minute ventilation, tidal volume, breathing and cardiac frequency were performed at rest in 51 patients with COPD while they breathed air and 44 oxygen, in a randomized double-blinded fashion. Results: Hyperoxia induced significant reductions in cardiac frequency and dyspnoea and a significant increase in SpO2. No significant change was noted in IC for the group overall, and there was substantial inter-subject variation in this measurement. No significant changes were found in ventilation, and there was no correlation between change in dyspnoea and change in IC. In patients with moderate to severe airflow obstruction (FEV1 <70 predicted), a significant association was found between the degree of airflow obstruction and change in IC induced by hyperoxia. Conclusions: Hyperoxia improved dyspnoea but did not significantly alter resting pulmonary hyperinflation in a group of patients with COPD of varying severity. However, in a subset patients with moderate to severe airflow obstruction a relationship existed between the severity of airflow obstruction and volume response to hyperoxia.