The gas mixture carbogen may be breathed by patients to enhance the oxygenation level and therefore the radiosensitivity of tumours. However, owing to the high CO2 content, its inhalation is associated with patient intolerance. Our aim was to determine a suitable carbon dioxide and oxygen gas mixture with similar enhancement of arterial oxygenation to 5% carbogen and with improved patient tolerance. 14 patients entered the study; of those 14, 8 were able to tolerate 2%, 3.5% and 5% carbogen mixtures as well as a control gas for sufficient time to allow successful arterial blood gas sampling. Gas exchange parameters were measured using a carbon dioxide monitor and a blood gas analyser. Arterial carbon dioxide tension ranged from 2.9 kPa to 6.82 kPa whilst breathing the carbogen mixtures, and arterial oxygen tension increased at least three-fold from basal values. There were no significant changes in the respiratory rate, heart rate and blood pH. The results suggest that 2% CO2 in O2 enhances arterial oxygen levels to a similar extent as 3.5% and 5% CO2 and that it is well tolerated.