Measurement of cardiac output with CO2 rebreathing method and thermodilution method was carried out in seven COPD patients undergoing mechanical ventilation. Mixed venous PCO2 could be measured reasonably well with CO2 rebreathing method. There was a very significant positive correlation between the CO2 method and thermodilution method for measuring cardiac output (r = 0.9130, P < 0.01). The change of cardiac output before and after infusion of sodium nitroprusside could be reflected by CO2 rebreathing method. We conclude that the cardiac output in COPD patients undergoing mechanical ventilation could be measured reasonably well with CO2 rebreathing method.