Congenital cystic adenomatoid malformation is a rare developmental abnormality of the lung. In most earlier reported cases, the anatomy of the bronchial tree was poorly documented. We describe four cases studied following autopsy. Post-mortem bronchography or serial microscopical examination showed segmental bronchial absence or atresia in each of them. Our observations provide further evidence pointing to bronchial atresia as being the primary defect leading to the development of congenital cystic adenomatoid malformation. The morphology of the lesion, i.e. the type of malformation, is determined by the extent of dysplastic lung growth beyond the atretic segment. The aetiology of the bronchial atresia is probably heterogeneous and may either represent a primary malformation, or be the result of vascular disruption.