Pressure controlled ventilation (PCV) is an alternative mode of ventilation which is used widely in severe respiratory failure. In this study, PCV was used for one-lung anaesthesia and its effects on airway pressures, arterial oxygenation and haemodynamic state were compared with volume controlled ventilation (VCV). We studied 48 patients undergoing thoracotomy. After two-lung ventilation with VCV, patients were allocated randomly to one of two groups. In the first group (n=24), one-lung ventilation was started by VCV and the ventilation mode was then switched to PCV. Ventilation modes were performed in the opposite order in the second group (n=24). We observed that peak airway pressure (P=0.000001), plateau pressure (P=0.01) and pulmonary shunt (P=0.03) were significantly higher during VCV, whereas arterial oxygen tension (P=0.02) was significantly higher during PCV. Peak airway pressure (Paw) decreased consistently during PCV in every patient and the percentage reduction in Paw was 4-35% (mean 16.1 (SD 8.4) %). Arterial oxygen tension increased in 31 patients using PCV and the improvement in arterial oxygenation during PCV correlated inversely with preoperative respiratory function tests. We conclude that PCV appeared to be an alternative to VCV in patients requiring one-lung anaesthesia and may be superior to VCV in patients with respiratory disease.