The purpose of this study was to review cases of congenital cystic adenomatoid malformations (CCAMs) arising in children and adults, in order to assess the recently expanded classification system for these lesions and their association with malignant transformation. Of 28 CCAMs, there were 16 type 1, 4 type 2, and 8 type 4 lesions, 12 of which presented in adults. Five of 16 type 1 CCAMs were accompanied by microscopic foci of bronchioloalveolar carcinoma; two others showed focal mucous cell hyperplasia. In two further cases, foci of nonmucinous atypical adenomatous hyperplasia were identified in the adjacent lung parenchyma. The bronchioloalveolar carcinomas showed less cytologic atypia, proliferative activity (Ki-67), and p53 expression than a comparative group of bronchioloalveolar carcinomas arising de novo, but this was not statistically significant (p = 0.15). Neither bronchioloalveolar carcinomas nor hyperplasia was identified in type 2 or type 4 CCAMs. Four of the eight type 4 CCAMs showed focal stromal hypercellularity, and one case subsequently developed a pleuropulmonary blastoma. We conclude that classification according to the current system is of clinical value. Bronchioloalveolar carcinomas arise in association with type 1 CCAMs, but recurrence following resection is exceptional. Type 4 CCAMs show histologic overlap with grade 1 pleuropulmonary blastomas, and distinction between these entities may not be possible on histology alone. However, stromal cellularity in a type 4 CCAM should raise the possibility of blastomatous transformation.