Alveolar stability was studied during prolonged enflurane anaesthesia by using a multiple inert-gas elimination technique for the assessment of the "continuous" distribution of ventilation-perfusion ratios (VA/Q). All 10 patients (mean age: 61 years, six smokers) presented with increased VA/Q mismatching during anaesthesia, with a redistribution of lung blood flow to regions with low or high VA/Q. Five patients had perfusion of units with VA/Q less than or equal to 0.07 which may cause unstable alveoli with the presently used inspiratory gas mixture. However, only two patients displayed increasing shunting suggestive of alveolar collapse during the 3.5 h observation period. This lower than expected incidence may indicate protective mechanisms against atelectasis, such as mechanical interdependence between lung units, or collateral ventilation.