Bronchogenic cyst. Bronchogenic cysts are thought to arise from abnormal budding of the ventral segment lung buds that give rise to the bronchial tree and lung. One hypothesis is that their position depends upon the timing of the developmental aberration that causes them. If it occurs early, then the cyst is located in the mediastinum (most commonly), but, if it occurs late, then the cyst is located within the lung tissue itself, most often in the lower lobes [30]. Another view is that case reports of intrapulmonary bronchogenic cysts probably represent instances of CPAM type 1 [23], although one series described 10 cases of pathologically verified intrapulmonary bronchogenic cyst [31]. Summary A new and expanded classification of congenital pulmonary airway malformations (CPAM)ofthelungidentifieslesions onthe basisofthelikelysiteoforiginandclinicaland pathological features. Within the five types, recent reports have demonstrated an association of CPAM type 1 with bronchioloalveolar carcinoma (BAC), and a degree of confusion in separating CPAM type 4 from the purely cystic form of pleuropulmonary blastoma. The CPAM type 1 lesion has been noted to contain clusters of mucogenic cells iny35% of cases, and these clusters are thought to predispose the patient to the development of BAC. The CPAM type 4 lesion, with its large and often thick-walled cysts lined with alveolar lining cells, may be confused with the cysts of type I pleuropulmonary blastoma, although its cyst walls contain subepithelial foci of rhabdomyosarcoma, almost exclusively underlying stretches of cuboidal-to-columnar epithelium, rather than the alveolar cell epithelium of CPAM type 4. Other cystic or pseudocystic lung lesions include post-infarction peripheral cysts resulting fromintrauterinepulmonaryarterythrombosis.Similarlyappearingcystshavebeennoted in Down’s syndrome. Air-filled cysts within the interstitium are a feature of acute and persistent interstitial pulmonary emphysema, and are limited to the interlobular septa. Fluid-filled cysts of congenital pulmonary lymphangiectasia are present within the interlobular septa, and extend laterally from the septa beneath the pleura. Congenital pulmonary lymphangiectasia is also frequently associated with congenital malformations of the heart. Bronchogenic cysts are rarely seen in infants, and are solid lesions usually separate from the lung. Extralobar sequestrations are also nonaerated lesions separate from the lung and occasionally found within or beneath the diaphragm. Intralobar sequestrations are usually acquired lesions (through infection), and may display air- or fluid-filledcysts,representingre-epithelialisedpost-infectiousabscesses.Finally,oneofthe mostcommonpulmonarylesionsininfantsandchildren,infantilelobaremphysema,isnot cystic but simply the overinflation of a segment of lung. Keywords: Bronchioloalveolar carcinoma, bronchogenic cyst, congenital pulmonary airway malformation, extralobar sequestration, insterstitial pulmonary emphysema, pleuropulmonary blastoma. J.T. STOCKER, A.N. HUSAIN 18