In 100 critically ill patients, intrapulmonary shunts (Qsp/Qt) calculated by assuming a carboxyhemoglobin (HbCO) of zero and a hemoglobin saturation (HbO2) derived from the Severinghaus nomogram were compared to shunts calculated utilizing measured values of HbCO and HbO2. The differences were statistically significant (p less than 0.001). These 100 patients had a mean Hb of 11.9 g/dl and a mean HbCO of 1.7%. Measured shunt calculations in 30 critically patients were prospectively compared with shunt calculations utilizing the mean assumed values derived from the 100 patients initially studies. No statistically significant differences occurred between these two shunts. It is, therefore, concluded that errors attributable to the common practice of assuming zero HbCO and deriving HbO2 from nomograms will produce falsely high calculated intrapulmonary shunt values in critically ill patients. It is, therefore, important to measure accurately HbCO and HbO2 concentrations when monitoring intrapulmonary shunting in critically ill patients. However, if such measurements cannot be obtained, utilization of an assumed value of 1.5% for HbCO and HbO2 nomogram values will minimize errors in the shunt calculation.