OBJECTIVES: To determine by 24-hour blood pressure monitoring the risk of hypertension late after coarctation repair in patients with arch hypoplasia. METHODS: Sixty-two of 116 consecutive patients (age, ≥10 years) who had coarctation repair and were quoted subjectively by the surgeon or the cardiologist to have arch hypoplasia at the time of the repair underwent a transthoracic echocardiogram and 24-hour blood pressure monitoring. Median age at repair was 11 days (range, 6-48 days). Mean preoperative z score of the proximal transverse arch was -2.43 ± 0.46. Eight patients had a repair via sternotomy (6 end-to-side anastomoses, 2 patch repairs) and 54 had a conventional repair via thoracotomy. RESULTS: After a follow-up of 18 ± 5 years, 27% of the patients (17/62) had resting hypertension and 60% (37/62) had abnormal ambulatory blood pressure. Sensitivity of high resting blood pressure in detecting an abnormal 24-hour ambulatory blood pressure was 41%. Twenty patients had arch obstruction at last follow-up. Eighteen of them (90%) had abnormal ambulatory blood pressure. None of the patients operated on with end-to-side repair via sternotomy had reobstruction compared with 33% (18/54) of those repaired via thoracotomy. CONCLUSIONS: Patients with a hypoplastic arch operated via thoracotomy have an alarming prevalence of hypertension. Regular follow-up with 24-hour ambulatory blood pressure monitoring is warranted, especially in patients who have had a smaller aortic arch at the time of the initial operation.