Pulmonary impairment is more frequent after cardiac surgery than after other major surgical procedures. The present study investigates whether, by using standard respiratory monitoring, i.e. side-stream spirometry and blood gas analysis, it is possible to detect changes in pulmonary function secondary to cardiopulmonary bypass. We investigated 18 patients undergoing elective coronary bypass surgery or aortic valve replacement. Cardiopulmonary bypass resulted in a nonsignificant increase in alveolar-arterial oxygen difference from 33.0 ± 10.6 kPa to 36.1 ± 12.5 kPa and arterial to end-tidal CO2 tension difference from 0.67±0.39 kPa to 0.79±0.54 kPa. Respiratory system resistance was unaltered. In contrast, dynamic compliance decreased significantly after cardiopulmonary bypass from 78.6 ± 22.9 to 65.4 ± 22.4 mL cmH2O−1 with open chest and from 61.0 ± 10.2 to 51.1 ± 17.2 mL cmH2O−1 with closed chest, compared with corresponding values before cardiopulmonary bypass. In conclusion, pulmonary gas exchange was not compromised after cardiopulmonary bypass, but a diminished respiratory compliance was a consistent finding, even in uncomplicated cardiac surgery using routine respiratory monitoring.