In the newborn infant, the diaphragm seems badly adapted to perform the burden of respiratory work. Indeed, due to the large angle of insertion on the rib cage and the small area of apposition, the flat diaphragm of the newborn infant seems better designed to suck in the rib cage rather than air. To better understand this paradox, and get insight in the structure-function relationship, the anatomical connections between the diaphragm and the rib cage were studied in 16 infants of various postmenstrual and postnatal ages. It was concluded (1) that the diaphragm inserts on the rib cage border only in the anterior costo-diaphragmatic triangle. From antero-laterally to posteriorly it inserts at increasingly greater distance from the rib cage border; (2) that the dorsal diaphragm ends its free course at the 11th rib and continues caudally as a spur ending between the 12th rib and the crista iliaca. From echographic studies of the right diaphragm with simultaneous measurement of the caudad displacement of the diaphragm and abdominal circumference change, the dynamics of the diaphragmatic movements could be better understood. It was concluded that, in contrast with the adult diaphragm, acting as a piston within the rib cage, the diaphragm of the newborn infant acts as a below moving mainly in the posterior part.(ABSTRACT TRUNCATED AT 250 WORDS)