Recoarctation is a problem in some patients after subclavian flap aortoplasty. To investigate the reason for recoarctation, we reviewed the records of 26 infants who underwent subclavian flap repair for symptomatic coarctation of the aorta at less than 3 months of age between June, 1979, and December, 1983. Age at repair ranged from 2 to 65 days (median 16 days) and weight from 2.1 to 4.9 kg (median 3.4 kg). In 14 patients the coarctation was associated with significant intracardiac defects (complex in six). There were two intraoperative deaths and one early death (surgical mortality 12%). The survivors were followed from 6 weeks to 66 months (median 12 months). Five survivors (22%), all operated on at less than 14 days of age, developed severe recoarctation 6 weeks to 6 months (median 5 months) after repair. The obstruction appeared to be due to lumen obliteration by shelf-life posterior wall tissue. Morphometric analysis of preoperative angiograms showed no correlation between recoarctation and distance between the left subclavian artery and the site of coarctation, length of the isthmus, diameter of the isthmus, combined cross-sectional area of the left subclavian artery and isthmus, or the ratio of the combined cross-sectional area of the left subclavian artery and isthmus to the cross-sectional area of the descending thoracic aorta. Recoarctation did not correlate with weight at operation, but it correlated significantly with age at aortoplasty (p = 0.02). The results suggest that intrinsic abnormalities of the periductal aortic wall are responsible for recoarctation after subclavian flap aortoplasty. Particular attention to this abnormal tissue at repair may prevent early recurrence in young infants.