Mesenchymal hamartomatous nodules and cysts in the lungs caused hemoptysis, pneumothorax, hemothorax, pleuritic chest pain, dyspnea of slight or moderate degree, or a combination of these signs and symptoms in five patients. In four cases the disease was multifocal and bilateral. The nodules were composed of primitive mesenchymal cells subdivided into papillae by a plexus of small airways lined with respiratory epithelium. The nodules grew slowly in number and size over the years and apparently became cystic when they reached a diameter of about 1 cm. The cysts had a cambium layer of mesenchymal cells and were lined with normal or metaplastic respiratory epithelium. In general, the disease had an indolent course. The most serious complications were sudden hemorrhage into a cyst from large systemic arteries supplying the walls of the cysts, pneumothorax or hemothorax from rupture of a subpleural cyst, and malignant transformation in one case. This disease appears to represent a distinct clinicopathological entity, which I term mesenchymal cystic hamartoma of the lung.