Lung cysts were observed by chance in the chest radiographs from two women aged 20 and 41. In the surgical specimens the lesions proved to be congenital cystic adenomatoid malformation type 1. The cysts were partially lined by mucous cells. Also near the cysts there were circumscribed tubuloacinar proliferations of mucous cells. In both cases a transition into a bronchioloalveolar carcinoma was seen. Histochemically no sulphomucins could be demonstrated by means of an alcian blue pH 1 reaction in the tumor cells, but was demonstrated in the non-neoplastic cells of the malformation. In both cases CEA was demonstrated in the tumor cells. Some cells in the tubuloacinar proliferations were weakly CEA positive. In one patient the diagnosis of carcinoma was made by intraoperative frozen section and a lobectomy was performed. The other patient had first only a resection of her cystic lesions and had to be reoperated because of the results of the pathological examination. Both patients had no recurrence in the 8 years following the operation. In the literature we found 5 cases of congenital cystic adenomatoid malformation in adults. In 2 cases there was also an associated bronchioloalveolar carcinoma. Several reports exist on the association of different kinds of cystic lung lesions and malignant tumors and their possible pathogenetic relationship. In this paper we draw attention to the development of malignant neoplasia in congenital cystic adenomatoid malformation in adults and its diagnostic problems.