Extralobar sequestration is defined as an isolated mass of pulmonary tissue that develops separately from the normal lung. It is invested in its own pleura and has a systemic vascular supply. Although usually discussed in parallel with intralobar sequestration because of the common finding of systemic arterial connection, it is useful to view extralobar sequestration as aberrantly located pulmonary mesenchyme arising from the primitive foregut, more analogous to bronchogenic cyst. Typically, extralobar sequestration is a separate, pleurally covered, wedge-shaped mass of lung positioned in the thoracic cavity below the normal lung, particularly on the left side (90% of cases). Other locations include the anterior and posterior mediastinum, within the diaphragm, intra-abdominal, or retroperitoneal. It may be associated with an ipsilateral diaphragmatic defect.

The arterial supply usually arises from the descending thoracic aorta, but it may arise from other systemic arteries. Venous drainage is variable and may be to either the pulmonary vein or a systemic vein. There is a vascular pedicle at the pole of the wedge closest to the midline, which usually contains an artery, a vein, and lymphatic channels. Typically there is no bronchus within this pedicle; if present, it may signal a connection to the digestive tract. Instead, there is an atretic bronchus in the proximal portion of the tissue mass, which is dilated and filled with mucus, similar to conventional bronchial atresia. Likewise, all cases show parenchymal maldevelopment, with about half of cases having microcystic maldevelopment similar to the small-cyst type of congenital cystic adenomatoid malformation (Stocker type II); others show features of pulmonary hyperplasia and may have lymphatic dilation. The parenchymal changes are part of the bronchial atresia sequence and do not reflect a separate process.