An air crescent sign describes the crescent of air that can be seen in invasive aspergillosis, semi-invasive aspergillosis or other processes that cause pulmonary necrosis. It usually heralds recovery and is the result of increased granulocyte activity.

In angioinvasive fungal infection, the nodules are composed of infected haemorrhagic and infarcted lung tissue. As the neutrophil count recovers and the patient mounts an immune response, peripheral reabsorption of necrotic tissue causes the retraction of the infarcted centre and air fills the space in between. This creates an air crescent within the nodules and is a good prognostic finding because it marks the recovery phase of the infection. This sign is seen in approximately 50% of patients.

It should not to be confused with the Monod sign 2 which describes the air that surrounds an aspergilloma. Unfortunately, the air around the fungal ball is also crescent shaped and the term air crescent sign is often used interchangeably in that instance. Opinion as to the validity of this latter use is controversial, but probably not worth losing too much sleep over.