A possible relationship between recurrence of coarctation and the presence of residual ductal tissue in the aorta was studied by defining the presence and pattern of ductal tissue in aortic segments from 23 patients less than 3 months of age who underwent resection of coarctation of the aorta. The surgical policy was to perform extensive excision of the coarctation, including a wide margin of descending aorta beyond the ductus arteriosus. Histologic examination showed that there was a circumferential sling of ductal tissue extending from the ductus arteriosus and surrounding the aorta at the level of the coarctation shelf in 22 specimens. In 15 of these specimens one or two tonguelike prolongations of ductal tissue extended distally from the circumferential sling and occupied constant positions in the aortic wall; one tonguelike prolongation extended distally below the insertion of the ductus and the other from the circumferential sling opposite the ductal insertion. Incomplete excision of ductal tissue was found in 11 specimens. In relation to the goal of avoiding recurrent coarctation after repair in the early months of life, the possible implications of these findings are discussed, and, in particular, a possible mechanism of recurrence after subclavian flap aortoplasty is proposed.