Pulmonary venous admixture (shunt) was measured in 10 mechanically ventilated patients with respiratory failure at varying oxygen tensions. All patients manifested a drop in pulmonary shunt as FIO2 was increased from 0.21 to 0.4. Further increase in FIO2 led to a gradual increase in the calculated venous admixture. The clinical significance of the conventional measurements of pulmonary shunt at FIO2 = 1.0 is questionable because of the marked effect of FIO2 on pulmonary shunt. Consequently, shunt measurements should be performed with the clinically useful FIO2.