Pulmonary gas exchange and the development of atelectasis were studied in eight essentially lung-healthy patients, awake and during halothane anaesthesia with mechanical ventilation. Gas exchange was evaluated by a multiple inert-gas elimination technique and conventional blood-gas analysis, and atelectasis was studied by computerized tomography (CT). Ventilation and lung perfusion were well matched in the majority of the patients when awake. In two patients there was low perfusion of poorly ventilated regions (low VA/Q). One patient had a shunt corresponding to 4% of cardiac output. None of the patients showed signs of atelectasis on the CT scans. After 15 min of anaesthesia, shunt had appeared in all patients, ranging from 1% in two patients (unchanged from the awake state) to 17%. The major VA/Q mode was widened and ventilation of poorly perfused regions (high VA/Q) was noted in seven patients. Densities in dependent lung regions (interpreted as atelectasis) were seen on the CT scans in six patients. The extent of atelectasis was significantly correlated both to the magnitude of shunt (r = 0.93, P less than 0.01) and to the impairment of arterial oxygenation (r = 0.99, P less than 0.001). The findings indicate that atelectasis in dependent lung regions during halothane anaesthesia creates shunting of blood flow and that atelectasis is the major or sole cause of impaired gas exchange in the lung-healthy, anaesthetized subject.