After repair of coarctation of the aorta using the technique of resection and end-to-end anastomosis, the internal diameters of the aortic isthmus and descending aorta often fail to increase. Better results seem possible with aortoplasty using the left subclavian flap technique. In order to clarify this matter, we investigated the structure of the left subclavian artery comparing it with that of the descending aorta and aortic isthmus: we studied the internal diameter, the thickness of the tunica media and the packing density of its elastic fibers in these vascular elements using a postmortem material of children with a coarctation of the aorta. The ages ranged from 4 days to 13 months with one child of 8 years. All 16 cases had one or more additional cardiac lesions. Operation had been performed in 3 children: 2 end-to-end anastomoses and one subclavian bypass of the aortic arch. Data were compared with observations on autopsy cases of children without cardiovascular abnormalities. The mean findings were that the calibers of the left subclavian artery and the descending aorta were within normal limits but that the caliber of the aortic isthmus was smaller than in normal children. The measurements on the tunica media showed that although, generally, the thickness of the media of the left subclavian artery was smaller than that of the aortic isthmus and descending aorta of the same individual, it contained relatively more elastic fibers than the matching vessels. This may indicate that the structure of the left subclavian artery is well suited to grow out as a part of the aortic arch.(ABSTRACT TRUNCATED AT 250 WORDS)