Prenatal detection and serial sonographic study of fetuses with lung lesions now make it possible to define the natural history of these lesions, determine the pathophysiologic features that affect clinical outcome, and formulate management based on prognosis. The prenatal diagnostic hallmarks and natural history of congenital cystic adenomatoid malformation of the lung and pulmonary sequestration are presented. The finding that fetuses with hydrops are at very high risk for fetal or neonatal death led to successful fetal surgical resection of the massively enlarged pulmonary lobe (fetal lobectomy) in several cases. The fetus with a lung mass but without hydrops has a good chance for survival with maternal transport, planned delivery, and immediate neonatal resuscitation and surgery. Large fetal lung tumors may partially disappear on serial prenatal sonography, suggesting that improvement can occasionally occur during fetal life.