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Localised coarctation of the aorta was studied histologically in 45 cases. The age range of the patients, from 2 weeks to 40 years, enabled us to make a comparison between early and late findings. The basic histological architecture of all coarctations was identical. The ridge consisted mainly of thickening plus some infolding of the aortic wall. Ductal tissue formed the inner part of the ridge for more than half of its total circumference. In older cases masses of secondary intimal proliferation were present, which narrowed the residual lumen of the coarctation. The coarctation was located preductally in most young patients and postductally in the majority of older patients. It is concluded that ductal tissue is invariably present in the ridge and that all coarctations develop in a preductal position. Secondary changes include a gradual shift of the ridge from a preductal to a postductal position, and progressive narrowing of the residual lumen by intimal proliferation.