To determine the effect of change in body position on gas exchange after thoracotomy, 12 patients with potentially resectable lung tumours were studied before and 24 hours after operation. Measurements of arterial blood gas tension (PaO2, PaCO2), alveolar-arterial oxygen difference (A--adO2), venous admixture effect (Qs/Qt percent), and physiological dead space to tidal volume ratio (Vd/Vt), were made in the supine, and left and right lateral decubitus positions. Preoperatively, altering position did not affect gas exchange significantly. After thoracotomy in the lateral position with the unoperated side dependent, PaO2 was significantly higher, and A--adO2 and Qs/Qt percent significantly lower than in the supine position. Postoperatively, the lateral position with the side of thoracotomy dependent was usually associated with the worst gas exchange. Only three patients achieved their best postoperative gas exchange in this position. In two this may have resulted from dependent small airway closure during tidal breathing, due to airways obstruction and old age, and in the third from postoperative atelectasis in this unoperated lung. No significant changes in mean PaCO2, Vd/Vt, or minute ventilation (VE) occurred with different positioning.