Arterial oxygen tension (PaO2) was determined on 49 patients within 24 hours of their admissions to a coronary care unit with chest pain. The response of the resting PaO2was tested to the use of 30 per cent oxygen (O2) by nasal cannula and additionally, in 14 patients, to the intravenous administration of furosemide. Twenty-two patients had acute myocardial infarcts (MI); 22, coronary insufficiency or unstable angina (CI); and five, nonischemic chest pain (NIP). Those with NIP had normal values. Most patients with ischemic pain had mild resting hypoxemia with no difference in values between the group having a MI versus those without MI. Although O2by nasal cannula raised PaO2to normal, significant pulmonary arteriovenous shunting (AVS) was suggested by the less than expected level of PaO2reached in both groups. Most patients given furosemide after O2raised their PaO2still further. The degree of hypoxemia and of AVS seemed related to clinical signs of heart failure although even those without overt signs demonstrated AVS. This study suggests that hypoxemia in acute ischemic heart disease is secondary to left ventricular failure. Thus, no distinction could be made on the basis of arterial hypoxemia and response to O2between the patients having an acute ischemic episode, with or without MI.