Atelectasis is the partial collapse or closure of a lung resulting in reduced or absence in gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct from pulmonary consolidation, in which they are filled with liquid. It is often referred to informally as a collapsed lung, although more accurately it usually involves only a partial collapse, and that ambiguous term is also informally used for a fully collapsed lung caused by a pneumothorax.

It is a very common finding in chest X-rays and other radiological studies, and may be caused by normal exhalation or by various medical conditions. Although frequently described as a collapse of lung tissue, atelectasis is not synonymous with a pneumothorax, which is a more specific condition that can cause atelectasis. Acute atelectasis may occur as a post-operative complication or as a result of surfactant deficiency. In premature babies, this leads to infant respiratory distress syndrome.

The term uses combining forms of atel- + ectasis, from Greek: ἀτελής, "incomplete" + Greek: ἔκτασις, "extension".

May have no signs and symptoms or they may include:

It is a common misconception and pure speculation that atelectasis cause postoperative fever. This claim has been perpetuated in medical textbooks as recently as 2021. Review articles published in 2011 and 2019 summarizing the available evidence on the association between atelectasis and post-operative fever concluded that there is no clinical evidence supporting this speculation. A recent article outlined the history of this myth and the true causes of postoperative fever.

The most common cause is post-surgical atelectasis, characterized by splinting, i.e. restricted breathing after abdominal surgery. Atelectasis develops in 75–90% of people undergoing general anesthesia for a surgical procedure.

Another common cause is pulmonary tuberculosis. Smokers and the elderly are also at an increased risk. Outside of this context, atelectasis implies some blockage of a bronchiole or bronchus, which can be within the airway (foreign body, mucus plug), from the wall (tumor, usually squamous cell carcinoma) or compressing from the outside (tumor, lymph node, tubercle). Another cause is poor surfactant spreading during inspiration, causing the surface tension to be at its highest which tends to collapse smaller alveoli. Atelectasis may also occur during suction, as along with sputum, air is withdrawn from the lungs. There are several types of atelectasis according to their underlying mechanisms or the distribution of alveolar collapse; resorption, compression, microatelectasis and contraction atelectasis. Relaxation atelectasis (also called passive atelectasis) is when a pleural effusion or a pneumothorax disrupts the contact between the parietal and visceral pleurae.

Risk factors associated with increased likelihood of the development of atelectasis include: the type of surgery (thoracic and cardiopulmonary surgeries), the use of muscle relaxants, obesity, high oxygen, the lower lung segments.