Arterial air embolism is an infrequent but often disastrous complication of various thoracic therapeutic procedures. Occasionally it may result paradoxically from air entering the systemic veins and reaching the systemic arteries through a septal defect, or possibly by other mechanisms. The serious manifestations are the result of obstruction by air bubbles of cerebral and coronary vessels, together with spasm of these vessels induced by the irritation of the gas. Animal experiments indicate that air introduced into the coronary circulation, either directly or by injection into the pulmonary vein or left auricle, produces ischemia of the myocardium in areas supplied by involved vessels. The ischemia is demonstrable grossly and by electrocardiographic examination. It may be very temporary, with speedy recovery, or it may persist even after the apparent complete disappearance of the gas bubbles. Death from ventricular fibrillation may result within a very short time after the injection of the air. The distribution of air within the arterial circulation is determined by the principle of air buoyancy. This principle may be made use of in the therapy of arterial air embolism.