Cystic lesions are used to describe inflatable cavernous lesions with distinct wall structures. The cystic lesions of the lung discussed herein include a group of diffuse lung diseases that resemble cystic changes. High-resolution CT (hrct) of the chest is useful in the differential diagnosis of this group of lesions. Common diseases characterized by cystic or cyst-like lesions include: lymphangioleiomyomatosis (lam), pulmonary histiocytosis x (plch), emphysema, common interstitial pneumonia (uip)/ idiopathic interstitial pulmonary fibrosis (ipf), and cystic bronchiectasis. Lymphangioleio-myomatosis (lam) is a form of diffuse parenchymal lung disease (dpld). It is characterized by abnormal proliferation of smooth muscle in the lymphatic vessels, airways, blood vessels and alveolar septa, and often leads to recurrent spontaneous pneumothorax, celiac disease, hemoptysis and cystic changes in the lungs, and eventually respiratory failure. The lesions are diffusely distributed in both lungs, with little difference between the upper and lower lung fields. The cystic wall is very thin and uniform, usually 5-15 mm in diameter, but larger cystic cavities are also seen, often formed by the fusion of multiple small cysts. Unlike emphysema, even though the cystic lesions in the lungs are very diffuse, the pulmonary vessels remain essentially normal without significant distortion. However, if the disease is terminal, the larger cystic cavities tend to fuse and the surrounding well-defined cystic walls are destroyed, making it difficult to differentiate lam from severe lobar central emphysema on hrct. lam is a thin-walled cystic lesion that sometimes cannot be detected on chest radiographs or routine chest CT, so hrct should be performed. hrct has a very high accuracy rate in diagnosing lam. lam differs from end-stage The difference between lam and end-stage IPF is that in the latter, the cystic cavity of the cellular lung is smaller and the wall is thicker, and the lesions are more often located in the periphery, mainly in the lower lobes. More than 90% of patients with pulmonary langerhans cell histocytosis (plch) have a history of smoking, and pediatric patients may have multisystem involvement of the skeleton, lungs, pituitary gland, and skin. Unlike lam, multiple irregularly shaped nodules and hollow nodules are seen, and the size of the nodules and hollow cavities are often heterogeneous, with no reduction in lung volume (Figure 2). A series of hrcts shows the progression of the disease, i.e., the progressive evolution of hollow nodules into cavities and their eventual fusion. These peculiarly shaped hollow nodules sometimes resemble dilated bronchi, but lack the continuity and segmentality of dilated bronchi. plch presents more often with pulmonary hypertension than chronic obstructive pulmonary disease (copd) or ipf. End-stage ipf may also have cystic lesions as the main manifestation. Cystic changes in ipf (honeycomb lung), with peripheral and reticulonodular shadowing, and distended bronchial dilatation are also seen. The lung volume is often reduced. If the lesion is distributed along the bronchovascular bundle, there are glassy changes, significant interlobular thickening, central lobular nodules and gas trapping, the diagnosis of ipf is not supported. hrct has a correct diagnosis rate of ipf of >90%. Patients who cannot be diagnosed by hrct are often in the early stages of the disease. Emphysema is often due to chronic obstructive pulmonary disease, i.e., it is often associated with smoking and environmental pollution. Emphysema is defined as “an abnormal, persistent enlargement of the terminal fine bronchi with destruction of the luminal wall without fibrosis, manifested by destruction and loss of the intrinsic structure of the alveoli”. This definition suggests that the disease should present with destruction of alveolar structure without fibrosis. The disease is commonly seen in smokers and presents with recurrent cough, sputum or dyspnea. Pulmonary function is characterized by obstructive ventilatory dysfunction and increased residual air volume/total lung volume. Hyperinflation of the lungs and diaphragm equalization are seen on chest radiographs, while sparse vascular texture on hrct is often the more characteristic manifestation of emphysema. In addition, the patient’s bilateral lung translucency is increased, and the wall of the sac is not clearly visible. The lesions are predominantly in the upper lungs or are diffuse in nature. It is important to note that emphysema can coexist with smoking-related interstitial diseases such as respiratory bronchitis with interstitial pneumonia (rb-ild) and plch, making the differential diagnosis difficult. Cystic bronchiectasis is a type of bronchiectasis that progresses from columnar and cystic bronchiectasis. Patients have a history of recurrent cough and sputum production. Pulmonary function also shows obstructive ventilatory dysfunction. The cystic wall is often thickened on hrct and has a clear distribution by lobe or segment. In severe cases, it is difficult to distinguish between dilated bronchi and cystic air spaces. However, the patient’s hrct often shows central bronchial dilatation, with the double-track sign and finger-loop sign being the most common (Figure 6), and cystic bronchial dilatation is the main manifestation in the advanced stage, which is difficult to distinguish from idiopathic bronchiectasis. Clinical attention should be paid to the presence of evidence of extra-pulmonary organ involvement, such as pancreatic exocrine insufficiency and abnormal sweat secretion, and the disease differs from idiopathic bronchiectasis in that it often starts in the upper lungs.