Acute laryngotracheobronchitis, also named croup, is due to parainfluenza virus infection. It usually affects children of 2-3 years old. Patients usually present with stridor.

Typical x-ray findings included steeple sign, causing narrowing below the lower margins of pyriform sinus, seen on frontal projections.

Narrowing of the upper trachea and distension of the hypopharynx are seen on lateral neck projections.