Changes of alveolar-arterial oxygen tension differences (AaDO2) after 35 cases of open heart surgery were evaluated with durations of extracorporeal circulation and prognosis. Following results were obtained. 1) AaDO2 values of pre-bypass in fatal open great cases were higher than in survival cases. 2) AaDO2 values of post-bypass were similar and noted no correlation to progonsis. 3) Luration of extracorporeal bypass time in fatal cases were longer than in survival cases. 4) Changes of AaDO2 values after extracorporeal circulation were much correlated to AaDO2 of pre-bypass than those of post-bypass.