Arterial and mixed venous oxygen tensions were measured in 24 patients following acute myocardial infarction while they were breathing air and 100% oxygen. Total venous admixture and the right-to-left shunt during 100% oxygen breathing were calculated. These data were related to the pulmonary arterial diastolic pressure, the cardiac index, and the central blood volume.Patients with myocardial infarction that was not complicated by congestive failure had blood gases, pulmonary shunts, and pulmonary arterial diastolic pressures comparable to control patients who were at rest in bed.When congestive failure complicated myocardial infarction, arterial blood oxygen tension was lower, pulmonary shunting was increased, and the pulmonary arterial diastolic pressure was elevated. Cardiac index and central blood volume were usually normal.The present data quantitate the contribution of anatomic shunting to the hypoxemia observed in myocardial infarction. Hypoxemia and increased anatomic shunting are closely correlated to the degree of elevation of pulmonary arterial diastolic pressure. The interrelationships of arterial hypoxemia, venous admixture, arterial-alveolar oxygen gradient, and pulmonary arterial diastolic pressure suggest that pulmonary venous congestion is an important determinant of the hypoxemia and shunting observed in patients with acute myocardial infarction.