Five children aged between 9 months and 7 years were admitted to hospital each with an unsuspected congenital diaphragmatic hernia. In 4 the diagnosis was pneumonia with a secondary pleural effusion or lung abscess. Initial investigations were unhelpful to the admitting physician; two of the children had had a previous chest x-ray which was normal. For 3 children the correct diagnosis was only made at necropsy. It is suggested that the possibility of a congenital diaphragmatic hernia be considered in any patient who has an indefinable diaphragm and cystic lesion on his chest x-ray film. Barium studies with the patient in Trendelenburg's position are of value in excluding the presence of bowel in the chest.