Sixteen children with congenital cardiac malformations were divided into cyanotic (n = 9) and acyanotic (n = 7) groups, and pulmonary ventilation and gas exchange were assessed before surgery, after sternotomy, just after the completion of cardiopulmonary bypass (CPB), 30 min after CPB and after closure of sternostomy before transfer to ICU. Most patients in the cyanotic group had oligaemic, while all in the acyanotic group had overperfused, lungs before surgery. Total compliance was similar in the two groups. Alveolar ventilation was higher and the physiological deadspace to tidal volume ratio (VD/VT) lower in the acyanotic group compared with the cyanotic group (P less than 0.05) before surgery. In the cyanotic group the preoperative large difference between arterial (PaCO2) and end-tidal (PE' CO2) carbon dioxide tension was smaller immediately after CPB and remained so after closure of the sternum. In the acyanotic children the small arterial to end-tidal carbon dioxide difference before CPB had increased just after and 30 min after CPB, and VD/VT was significantly increased (P less than 0.05) just after CPB. After closure of the sternum the arterial to end-tidal carbon dioxide difference and the VD/VT ratio had returned to baseline. The arterial to end-tidal carbon dioxide difference is a good indicator of ventilatory efficiency after open heart surgery.