Respiratory mechanics were studied in five anaesthetised children, aged 3 to 33 months undergoing urological surgery, in both the supine position and with extreme truncal flexion. Extreme truncal flexion was associated with a reduced respiratory system compliance. Dynamic compliance decreased significantly, by 30% (range 12-55%) and static compliance decreased significantly, by 40% (range 18-65%). There were no changes in respiratory system resistance. Tidal volume was also significantly reduced (mean 20%) despite a significant increase (mean 22%) in peak ventilator pressure. These changes in mechanics must be recognised to avoid alveolar hypoventilation, with a consequent decrease in gas exchange during surgery.