In a group of 18 male patients undergoing coronary artery bypass grafting with cardiopulmonary bypass, the overall incidence of post-operative atelectasis was 60%. Nearly three-quarters occurred during anaesthesia. After operation there was no difference whether CMV or IMV without PEEP was provided overnight. Atelectasis already present did not improve and further atelectasis occurred. A role for IMV is not excluded, since it facilitates the use of PEEP. Many factors operate and interact to provoke atelectasis during anaesthesia, which increases post-operative morbidity. Many of these factors are prevertible or reversible if their physiological basis is understood. Optimal post-operative ventilation should be tailored to the needs of the individual patient and demands close co-operation between anaesthetist and surgeon.