Hypoxaemic patients with exacerbations of chronic obstructive pulmonary disease (COPD) are at some risk of carbon dioxide (CO2) retention during oxygen therapy. We quantified the risk of CO2 retention with oxygen therapy in COPD in 24 consecutive patients presenting to the accident and emergency department with acute exacerbations associated with hypercapnic respiratory failure (partial arterial pressure of oxygen [PaO2] < 8 kPa and partial pressure of CO2 [PaCO2] > or = 6.5 kPa). Only three patients developed clinically important CO2 retention (defined as a rise in PaCO2 > 1 kPa) with controlled oxygen therapy (24-40% by Venturi mask to maintain the oxygen saturation at 91-92%). These patients presented with more severe hypercapnia, but all three required only low-flow oxygen (24-28%). These findings suggest only a small risk of aggravating hypercapnia with controlled oxygen supplementation.