In utero evolution and postnatal outcome were studied for 18 cases of congenital adenomatoid malformation (CAM) diagnosed by prenatal ultrasound. Five were macrocystic CAM, 9 were microcystic, and 4 were homogeneously hyperechogenic. Three fetuses presented with hydrops. Pulmonary amniotic shunting was performed in 3 patients. Outcome was the following: 4 were aborted, 1 died neonatally, and 13 survived. Four of these infants required no surgery in the neonatal or postneonatal period. In three of these, the size of the mass had decreased spontaneously in utero. Outcome did not appear to be related to the anatomic type of CAM nor to the presence of moderate polyhydramnios, but was related to the degree of mediastinal compression and to the existence of hydrops. A clearer understanding of the natural evolution of CAM is useful to determine the indications for in utero therapy.