This study was designed to determine, first, the stiffness of the pulmonary arteries and, second, the relationship between the stiffness of the pulmonary arteries and the success rate of balloon angioplasty in patients with complete transposition after an arterial switch operation. Indexes of pulmonary arterial wall stiffness, percent change in the radius during a cardiac cycle and the pressure elastic modulus, were calculated from the pulmonary arterial pressure and radius measured from a cineangiogram. Of the patients, 13 had no significant stenosis, while 25 had significant stenoses and, therefore, underwent balloon angioplasty. In all, we dilated 33 stenotic lesions. In the presence or absence of postoperative pulmonary stenosis, the percent change in the radius of the pulmonary trunk was significantly less than the normal value. The pressure elastic modulus of the trunk and its branches was significantly greater than normal, and increased significantly with time after the operation. In patients without pulmonary stenosis, the systolic pulmonary arterial pressure increased concomitantly with the pressure elastic modulus. Balloon dilation was successful at 17 locations and unsuccessful at 16 locations. The percent change in radius was significantly less for the failures than for the successes. Prior to 3.5 years after the operation, the pressure elastic modulus was < 400 g/cm2 at most locations and balloon dilation was successful in 88%. More than 3.5 years after the operation, the pressure elastic modulus was < 400 g/cm2 in 11 of 15 locations, and balloon dilation was successful in only one location. The increased stiffness of the pulmonary artery may result in increased pulmonary systolic pressure in patients without pulmonary stenosis, and result in a low success rate for balloon angioplasty in patients with pulmonary stenosis.