A modified knee-elbow position which facilitates the repair of vesico-vaginal fistulae is described. Some respiratory effects consequent on the assumption of this position were investigated. In twenty-seven conscious male subjects the vital capacity was reduced by an average of 16.3 per cent on assuming the position. In twelve patients under light anaesthesia with halothane there was no difference in mean values for respiratory rate, minute volume, arterial carbon dioxide tension and oxygen saturation obtained in the modified knee-elbow and in the supine position. It is concluded that the use of this position in patients with healthy lungs does not increase the hazards of the operation if respiratory muscle function is not impaired by deep anaesthesia or the injudicious use of muscle relaxants.