As mentioned above, the five types of CPAM are thought to arise from arrested development at various stages in the formation of the fetal bronchial tree. It is important to distinguish between each type, given the variable association with both malignancy and other congenital disorders. Type 0, the tracheobronchial subtype, is the rarest and is seen in < 2% of all described cases. It is commonly associated with congenital cardiovascular abnormalities or renal hypoplasia. On gross examination, the lung is small and firm with a diffusely granular surface. Histology reveals multiple irregular bronchiole-like structures, many of which are surrounded by thick cartilaginous plates and bundles of smooth muscle. Vascular structures are often > 100 μm away from these bronchiole-like structures, making gas exchange impossible. Unless the fetus is placed on extracorporeal membrane oxygenation at delivery, death is immediate. Type 1, the bronchiole/bronchiolar subtype, has already been described above. Type 2, the bronchiolar subtype, is composed grossly of small cysts < 2 cm in diameter, with involvement usually of only one lobe. It is also commonly associated with other congenital abnormalities such as renal agenesis. On microscopic examination, back-to-back bronchiolar structures lined by low columnar cells blend in with normal alveolar structures. Type 3, the bronchiolar/alveolar duct subtype, often involves the entire lung unilaterally. On histologic investigation, scattered bronchiolar or alveolar duct-like structures lined by a low cuboidal epithelium are seen. There is a characteristic absence of pulmonary vasculature. Type 4, the distal acinar subtype, comprises 10% of all described CPAMs. It grossly resembles the type 1 CPAM with large peripheral cystic lesions. However, on microscopic examination, the cysts are lined by both type 1 and type 2 pneumocytes. It is important to distinguish type 4 CPAM from pleuropulmonary blastomas, as the cystic form of the latter can very closely resemble CPAM type 4. Prior case reports12,13 linking CPAM type 4 and pleuropulmonary blastoma are likely erroneous, given the morphologic similarities between the two lesions.