Twenty cases of cystic adenomatoid malformation of the lung were observed: 2 had died in utero; the diagnosis was made at birth in 13 infants of which 3 were premature. A Bochdalek's hernia had been diagnosed before birth in 2 cases by echo-tomography. The correct antenatal diagnosis had been made in 2 cases. Two infants had no symptoms, 3 were dyspneic, 8 were in respiratory distress and had to be intubated and ventilated. Two had a prune belly syndrome. Eight infants had a thoracotomy during their first week of life. Pulmonary resections concerning those 13 patients comprised 10 lower lobectomies, 1 of which was associated with a lingulectomy, and 2 upper lobectomies, 1 of which was associated with a middle lobe resection. Five patients were diagnosed and operated upon between 10 months and 8 years of age; 4 had recurrent bronchitis and 1 was diagnosed during the treatment of a gastroenteritis. They had 3 lower and 2 upper lobectomies. Recovery was uneventful in all patients except for 1 who was reoperated upon for intestinal obstruction. Antenatal diagnosis of cystic adenomatoid malformation should become standard. The malformation may be mistaken for a pulmonary sequestration or bronchogenic cyst. Differential diagnosis of a congenital hernia is important.