This retrospective study attempts to assess the size and growth pattern of the pulmonary artery about 1 year after neonatal arterial switch operation for simple transposition of the great arteries. Sixty-seven patients underwent cardiac catheterization, including catheterization of the right and left pulmonary arteries, and right ventricular angiography an average of 13.9 months after arterial switch operation. In 34 of these patients pre-operative right ventricular angiocardiograms were available. The diameter of the main pulmonary artery and that of its proximal right and left branches were measured. The values were compared to those of normal children matched for body surface area, taken from the literature and, for the branch values, related to the degree of branch stenosis and to the corresponding values, measured on pre-operative angiocardiograms. The cross-section of the main pulmonary artery after arterial switch operation with Lecompte manoeuvre becomes oval. The branches of the pulmonary artery are sometimes underdeveloped and this finding is related to branch stenosis. The ratio of the branch gradients is inversely proportional to the growth ratio of both branches while the pre- and postoperative Nakata indices are identical. Conclusion The Lecompte manoeuvre induces a flattening of the main pulmonary artery with concomitant reduction of its cross-sectional area. There is also frequently trivial or rarely moderate branch pulmonary stenosis which is accompanied by growth retardation of the concerned branch.