We have confirmed the value of measurement of end-tidal carbon dioxide concentration as an indicator of arterial carbon dioxide tension during the use of the laryngeal mask airway in healthy patients breathing spontaneously. The mean difference between arterial and end-tidal carbon dioxide tension was 0.52 kPa (range 0-1.5 kPa), which is similar to the difference which has been reported when a tracheal tube has been used.