Cystic adenomatoid malformations and sequestrations of the lung are uncommon but potentially devastating problems of the fetus and neonate. We have followed over 50 cases of fetal lung masses from the time of prenatal diagnosis. Serial prenatal ultrasonography demonstrated that 9 large pulmonary lesions dramatically decreased in size or disappeared completely. We conclude that the natural history of prenatally diagnosed fetal lung masses is highly variable. A huge mass associated with fetal hydrops has a dismal outcome. If hydrops is not present, then the initial impression concerning prognosis may not accurately predict outcome, because there may be marked improvement during fetal life.