Mucinous cystadenoma of the lung. A report of two cases with immunohistochemical and ultrastructural analysis. We describe two patients who presented with solitary pulmonary masses that consisted of unilocular cysts lined by columnar mucinous epithelium. The cysts contained copious mucus. The epithelial lining of the cysts showed foci of stratification and papillary infolding. Histologically identical lesions have previously been termed unusual mucous cysts or mucinous cystadenomas. We believe that these tumors are true neoplasms differentiating toward the respiratory epithelial mucous cell. They should be distinguished from a variety of pulmonary neoplasms including bronchoalveolar carcinoma, bronchial mucous gland adenoma, mucoepidermoid carcinoma, and metastatic adenocarcinoma. 