Two cases of pulmonary blastoma are reported. One occurred in a 29-year-old man 31 months after a cyst had been excised from the same pericardial-pleural area. The cyst appeared to be of mesothelial origin and, although supported by a cellular stroma, was originally considered to be benign. The morphologic features, location, and possible relationship to a mesothelial lesion suggest an embryonal-mesothelial origin of the tumor. The patient died of recurrent and metastatic disease 9 years after the first resection. The second case is that of a now 74-year-old woman who, over a period of 19 years, has had six separate small peripheral lung tumors excised. They originally resembled fibrous hamartomas but included embryonal-type immature areas and became increasingly more cellular and sarcomatous. The fine structure of the recurrent tumors is that of primitive stroma with few fibrils but no other differentiating features.