The arterial hypoxemia frequently present in acute myocardial infarction was compared with simultaneous clinical examination on 177 occasions throughout 40 episodes of infarction and recovery. The arterial oxygen tension and the increment with 28 per cent oxygen were found to correlate with clinical severity. The daily arterial carbon dioxide, pH, and lactate studies did not accurately reflect changes in clinical course. The complex physiologic derangements are discussed. Arterial blood gas analysis is a safe and useful tool for evaluating the clinical course and management of acute myocardial infarction.