Swyer-James Syndrome, originally described on the basis of the plain film radiographic findings alone, is also known as "unilateral (or lobar) emphysema" or "unilateral hyperlucent lung". The pathology was originally described by E. M. Macleod. It is for this reason that radiologists often refer to the syndrome as Swyer-James and pathologists often refer to the same syndrome as Macleod.

The syndrome is a chronic complication of bronchiolitis during childhood. Although a variety of organisms may cause the infection, adenovirus is the most common. The infection itself may be asymptomatic, but more often the child has recurrent pulmonary infections and eventually develops bronchiectasis. Symptoms of the syndrome may thus be those of a bronchiectasis, including productive cough, dyspnea and occasional hemoptysis. Most patients, however, have minimal symptoms that are not specific for bronchiectasis. This is especially true when the degree of bronchiectasis is minimal or mild, radiographically and pathologically.

The classic plain film findings are hyperlucency of one lung with diminished or normal lung volume on that side and diminished number and size of pulmonary vessels in the hyperlucent areas. The central portion of the pulmonary artery on the hyperlucent side may be diminished in size but is always visible. Evidence of air trapping in the hyperlucent lung is usually present only during expiration, so that expiratory films are often extremely useful for the diagnosis. Ventilation-perfusion scans show matched defects of ventilation and perfusion in the hyperlucent regions.

Computed tomography, especially high resolution CT of the lung (HRCT), is the most useful modality in most cases of Swyer-James Syndrome. The pulmonary vessels present in the hyperlucent areas are diminished in size, and there is nearly always evidence of bronchiectasis.

The differential diagnosis includes congenital abnormalities of the pulmonary artery, especially pulmonary artery atresia; this is uncommon in an adult presentation, and the central pulmonary artery is extremely diminutive. Localized bullous emphysema produces deviation of vessels with the absence of vessels in the most lucent regions of lung. HRCT is excellent for demonstration of these findings. Complete obstruction of one or more bronchi may present with plain film findings identical to Swyer-James Syndrome; and, again, HRCT or bronchoscopy are often necessary to eliminate this possibility. In the past, bronchography was used for this differential diagnosis. Bronchographic findings of Swyer-James Syndrome are those of bronchiectasis with sharply terminating bronchial segments.