We describe 25 cases of echogenic or complex fetal lung masses seen sonographically and suspected of being cystic adenomatoid malformations or sequestrations of the lung. On the basis of prenatal sonographic appearance, 40% of fetuses were suspected of having type 1, 20% type 2, and 40% type 3 cystic adenomatoid malformations or sequestrations. Sixteen (64%) of the 25 fetuses with lung masses survived the perinatal period, and 80% of the nonaborted fetuses survived. Eleven infants (69% of liveborns) had no respiratory symptoms at birth. Nine of the survivors underwent surgical resections of their masses after birth, whereas the other seven children are being followed conservatively. The postnatal diagnoses revealed that four of the fetuses had sequestrations, one had a sequestration with elements consistent with cystic adenomatoid malformation, and two who were thought to have type 1 cystic adenomatoid malformation had an esophageal duplication cyst and a thoracic neuroblastoma, respectively. All the other infants who had a pathologic diagnosis or postnatal imaging had cystic adenomatoid malformations. Increasing mediastinal shift was associated with decreasing survival as 90% of fetuses with no mediastinal shift are alive, whereas 50% of the nonaborted fetuses with a severe mediastinal shift survived. Follow-up scans in utero were available in 15 cases. The size of the mass became smaller in 53% during gestation. Seventy-one percent of pregnancies had normal amniotic fluid volumes and 29% were complicated by polyhydramnios. Survival of nonaborted fetuses was 100% in pregnancies with normal amniotic fluid compared with 50% in those with polyhydramnios. Eight percent of the fetuses with chest masses had additional structural abnormalities and were karyotypically abnormal. In conclusion, many fetuses with lung masses show improvement of the sonographic findings in utero, and many infants may not be symptomatic at birth. Of the survivors in this series, only slightly greater than half underwent surgery.