The effect of general anaesthesia on postoperative pulmonary mechanics was studied by comparing pre- and postoperative spirometry in 30 patients undergoing extracorporal shock wave lithotripsy, 15 of whom received a unilateral intercostal nerve block, the remainder a general anaesthetic. Both groups of patients had significant postoperative changes in peak expiratory flow rate, expiratory reserve volume and vital capacity; forced expiratory volume at one second was not significantly changed in either group. There were no significant differences in the pre- or postoperative mechanics between the two groups. We conclude that general anaesthestics do not play a significant role in the generation of abnormalities in postoperative spirometry.