Atelectasis is defined as the collapse or closure of the lung resulting in reduced or absent gas exchange. It may affect part or all of one lung. It is a condition where the alveoli are deflated, as distinct from pulmonary consolidation. It is a very common finding in chest x-rays and other radiological studies. It may be caused by normal exhalation or by several 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 features atelectasis. Acute atelectasis may occur as a post-operative complication or as a result of surfactant deficiency. In premature neonates, this leads to infant respiratory distress syndrome. Symptoms: cough, but not prominent; chest pain; breathing difficulty; low oxygen saturation; pleural effusion (transudate type); cyanosis (late sign); increased heart rate; low-grade fever; It is a common misconception (especially among surgeons) that atelectasis causes fever. A study of 100 post-op patients followed with serial chest X-rays and temperature measurements showed that the incidence of fever decreased as the incidence of atelectasis increased.