Chest X-rays were analysed in 22 male and 16 female patients (mean age 30.7 years) with coarctation of the aorta. The results were correlated with angiography, haemodynamic pressures and operative findings. The left subclavian artery was prominent in 33 cases, signs indicating a collateral circulation (rib notching, internal mammary artery) were present in 26 cases. In addition there were changes of cardiac and aortic configuration. The least reliable sign was constriction of the aorta due to the stenosis itself. Frequent combinations of X-ray signs were dilatation of the left subclavian artery and a collateral circulation, in addition to signs of increased pressure (n = 25). Less frequently (n = 8) only a dilated subclavian artery and signs of increased pressure were found. The prominence of the subclavian artery was particularly marked in systolic pressure gradients through the stenosis of greater than 40 mm Hg, and the formation of a collateral circulation when average aortic pressure differences above and below the stenosis were less than 40 mm Hg. Collaterals were to be found most frequently with a correlation of 2:1 of the systolic gradient to the mean pressure gradient delta Ps/delta Pm. A very good correlation existed comparing X-ray signs with findings at operation. The diagnosis of coarctation of the aorta can thus be made with sufficient certainty from the plain chest X-ray.