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A pulmonary cyst may be defined as an abnormal pulmonary parenchymal space, not containing lung but filled with air and/or fluid, congenital or acquired in etiology, typically with a circumscribed interface with adjacent normal lung, and a wall thickness >1 mm. Pulmonary cysts are not infrequent abnormalities at thoracic computed tomography. Occasionally, pulmonary cysts are incidentally encountered at thoracic imaging studies and, when limited in number, may even be regarded as a consequence of normal age-related pulmonary changes. The causes of cysts within the lung parenchyma are numerous, and pulmonary cysts should be distinguished from other causes of focal lucent pulmonary lesions, including emphysema, bronchiectasis, air trapping, and cavitary lung disease. Diffuse pulmonary cystic diseases constitute a distinct subset of causes of pulmonary cysts and include such disorders as Langerhans cell histiocytosis, lymphangioleiomyomatosis, Birt-Hogg-Dubé syndrome, and lymphocytic interstitial pneumonia, among other even less common entities, including etiologies of small airway obstruction, such as asthma and bronchiolitis obliterans, and even amyloidosis or light-chain deposition disease. Often, the clinical history and presentation and the imaging appearances of the cysts may allow a specific diagnosis to be offered, but there is overlap among the imaging appearances of several of the causes of diffuse cystic pulmonary disease, and therefore tissue sampling is frequently required to establish the correct diagnosis.