To evaluate the clinical characteristic of pulmonary gas exchange in patients with low pulmonary capillary wedge pressure (PCW less than 18 mmHg) in the early phase of acute myocardial infarction, we investigated 48 patients admitted within 24h from the onset of chest pain. The alveolar-arterial oxygen tension difference (AaDO2) and arterial blood gas parameters were obtained while breathing room air on admission. The disturbance of pulmonary gas exchange, as revealed by the high value of AaDO2, existed without left ventricular dysfunction, and AaDO2 had no significant relationship with any of the hemodynamic parameters including the difference between plasma colloid osmotic pressure and PCW. Multiple regression analysis was performed using 9 variables related to pulmonary gas exchange, to identify those which were important. As a result, QRS score (p less than 0.01) and age (p less than 0.05) were identified as significant variables related to AaDO2. Thus, the disturbance of pulmonary gas exchange in patients with low PCW was caused by mechanisms other than left ventricular failure which may reflect the extent of ischemic myocardium.