__________________________________________________ ___To assess the consequences of myocardial infarction on the lung, pulmonary gas exchange and distribution of pulmonary perfusion and ventilation were studied after uncomplicated myocardial infarction in 35 patients in whom no clinical or radiographic evidence of congestive failure was apparent. Studies were performed in some of the patients during the first and third weeks after infarction and in 7 patients 2 to 6 months later.During the first week, the mean arterial oxygen tension was decreased to 72 mm Hg and the mean alveolar-arterial oxygen tension gradient increased to 47 mm Hg; both values improved in the convalescent phase. The mean arterial oxygen tension was 447 mm Hg and the mean alveolararterial oxygen tension gradient was 203 mm Hg while breathing 100 per cent oxygen during the first week after infarction; both values returned to normal in the 2-to 6-month follow-up studies. Distribution of pulmonary perfusion was abnormal in the first week. There was diminution of the normally occurring vertical gradient of perfusion in the lung, with relative hypoperfusion of lung base, increased perfusion of lung base, and increased perfusion of the apex. This abnormality returned toward normal in the follow-up period.No other hemodynamic abnormalities were demonstrated. The underlying mechanism of both hypoxemia and perfusion maldistribution appeared to be fluid accumulation in the lung interstitium secondary to transient left ventricular failure at the time of acute myocardial infarction.