Thirty-seven critically ill infants and children were seen over a ten-year period with the diagnosis of complete "D" transposition of the great arteries. Since 1967, balloon atrial septostomy has been performed at the original diagnostic cardiac catheterization. Previously, palliative treatment was surgical. Considerable improvement immediately followed balloon septostomy, and eight of the ten infants seen since 1967 are alive and doing well. Death occurred following attempted banding of the pulmonary artery in two infants with ventricular septal defects. One, a 7-day-old, also had an associated coarctation of the aorta. The use of balloon septostomy has greatly improved the outlook for infants with transposition of the great arteries, and allows more of them to grow to an age and size when total physiological correction (Mustard operation) may be safely undertaken.