In 1985, Dr. Göran Hedenstierna pioneered in the transition of atelectases during anaesthesia from a concept to a clinical entity, using computed tomography to detect "pulmonary densities". These densities were soon to be fully recognized as atelectasis. Most of the conclusions in the original paper are almost 40 years later still scientifically intact: the immediate appearance of atelectasis after induction of anaesthesia in the majority of adult patients, that atelectasis impedes arterial oxygenation by shunting deoxygenated blood, and the efficacy of a positive end-expiratory pressure to oppose atelectasis. The importance of atelectasis in the development of postoperative pulmonary complications is still obscure, as is the concept of protective ventilation. A common denominator in several studies on protective ventilation during anaesthesia is the lack of recognising the importance of the oxygen concentration. The pivotal role of oxygen in the development of atelectasis, and the impact of oxygen in relation to different conditions in the lungs, needs further studies.