Surgical indication for aortic arch hypoplasia in infants. To determine the surgical indications for aortic arch hypoplasia, the distal arch outer diameter, distal arch index (ratio of the distal arch diameter to the normal aortic ring diameter), and postoperative pressure gradient across the aortic arch were studied in 23 patients under 6 months of age who underwent surgery for coarctation and/or aortic arch hypoplasia. The ratio of the pressure gradient across the arch to the right radial artery was used to evaluate the postoperative level of stenosis. The maximum ratio of the pressure gradient that could be tolerated after surgery was considered to be 0.15 from the operative results. Negative correlations were found between the distal arch outer diameter and postoperative pressure gradient ratio (r = 0.80), and the distal arch index and postoperative pressure gradient ratio (r = 0.80). These correlations proved that in order to obtain a postoperative pressure gradient ratio of 0.15 or less, a distal arch outer diameter of 3.9 mm or more and a distal arch index of 0.63 or more were necessary. Consequently, a distal arch outer diameter of 3.9 mm or a distal arch index of 0.63 is considered to indicate that aortic arch hypoplasia is in need of repair. 