Pulmonary function was studied in thirty-five patients before, during, and immediately after major orthopaedic operations. One group of twenty patients had corrective procedures for idiopathic scoliosis. Fifteen patients had operations involving the extremities. In the patients with scoliosis we found that pulmonary mechanics, as expressed by flow resistance and elastic compliance, deteriorated during the operation; that pulmonary right-to-left shunting increased and arterial oxygen tension on air decreased immediately postoperatively; and that vital capacity was greatly reduced after the operation. No significant changes took place in the rest of the patients. We concluded that corrective operations for idiopathic scoliosis are associated with major reductions of pulmonary function in the immediate postoperative period, so that postoperative respiratory failure would be a risk in patients who had compromised pulmonary function preoperatively. We therefore recommend pulmonary function testing preoperatively and outline one approach to this problem.