The incidence and the outcome of systemic air embolism in 340 consecutive patients who underwent cardiac surgery under cardiopulmonary bypass in this unit for congenital defects of the cardiac septa and diseases involving the aortic and mitral valves have been studied. This was thought to have occurred in 40 patients, of whom 10 died. The distribution of air embolism according to the types of operation undertaken was as follows: six of 127 for atrial septal defect; six of 36 for ventricular septal defect; seven of 42 for mitral valve replacement; seven of 47 for aortic valve debridement; and 14 of 55 for aortic valve replacement. The cause was considered to have been systolic ejection of air into the aorta which, following cardiotomy, had been trapped in the pulmonary veins, the left atrium, the ventricular trabeculae, and the aortic root. Since the adoption of a more rigid `debubbling' routine, air embolism has not occurred. The incidence of pulmonary complications occurring in these patients after bypass was studied. Unilateral atelectasis, which occurred in five patients, resulted from retained bronchial secretions in all and was cured by bronchoscopic aspiration in all. The cause of bilateral atelectases, occurring in nine patients and fatal in eight of these, appeared to be related to cardiopulmonary factors and not to air embolism. Acute air injection made into the pulmonary artery of a dog resulted in pulmonary hypertension and a grossly deficient pulmonary circulation, but changes were largely resolved within a week. In view of this, it is considered that pulmonary air embolism may temporarily embarrass the right heart after the repair of a ventricular septal defect in a patient with an elevated pulmonary vascular resistance and diminished pulmonary vascular bed.