In 11 patients requiring coronary bypass surgery during extracorporeal circulation we measured pulmonary function and hemodynamic variables before surgery and during the postoperative period. None of the patients presented with pulmonary hypertension or cardiac failure. Cardiac surgery with extracorporeal circulation caused an important decrease in functional residual capacity (-60%) and in arterial oxygenation (-44%). In the immediate postoperative period, the addition of a positive end-expiratory pressure (PEEP) to mechanical ventilation produced an increase in functional residual capacity and in arterial oxygen tension, and a slight decrease in cardiac output. In contrast to the results obtained in children after cardiac surgery and in adults presenting with viral pneumonia or respiratory distress after polytrauma or surgery, continuous positive airway pressure during spontaneous breathing (CPAP) did not improve pulmonary gas exchange in our patients, despite the significant increase in functional residual capacity produced. This suggests that CPAP did not cause recruitement of atelectatic areas or that this effect was offset by a parallel overdistension of gas exchange units.