Swyer-James syndrome is a rare condition in which the lung (or portion of the lung) does not grow normally and is slightly smaller than the opposite lung, usually following bronchiolitis in childhood. It is typically diagnosed after a chest X-ray or CT scan which shows unilateral pulmonary hyperlucency (one lung appearing less dense) and diminished pulmonary arteries. Affected individuals may not have any symptoms, or more commonly, they may have recurrent pulmonary infections and common respiratory symptoms. The cause of the condition is not completely understood.

The cause of Swyer-James syndrome is not completely understood. Most experts agree that the initial abnormality occurs in the distal bronchi (air tubes that bring air to and from the lungs) after an infection during early childhood. The smaller size of the affected lung may be due to the infection inhibiting the normal growth of the lung. A number of reports have described Swyer-James syndrome following childhood histories including radiation therapy; measles; pertussis (whooping cough); tuberculosis; breathing in a foreign body; mycoplasma; and viral infections, especially adenovirus. Research has suggested that a hyper-immune reaction in the lung (producing an unusual abundance of antibodies) may play a role in sustaining airway damage after the initial infection. Some have argued a pre-existing lung abnormality may predispose individuals to the condition. Although bronchial damage of some kind during childhood is generally considered to play an important role, many affected individuals have had no known history of an airway infection. It is possible that some unknown factors present at birth may contribute to the development of Swyer-James syndrome.

Individuals with Swyer-James syndrome may not have any symptoms, but affected individuals can have chronic or recurring lung infections, shortness of breath (dyspnea) when performing an activity, coughing up of blood (hemoptysis), and even severe respiratory impairment.

It has been proposed that individuals with Swyer-James syndrome may benefit from lung volume reduction surgery (LVRS), a procedure in which damaged tissue is removed from the lung. LVRS was reportedly performed successfully in an individual with Swyer-James syndrome, and it has been suggested that the procedure could be used for managing the condition in other affected individuals because it has shown to be effective for improving pulmonary function and symptoms.