OBJECTIVES: Branch pulmonary artery (BPA) size is one of the factors that influence the efficacy of the Fontan circulation. Central pulmonary artery stenosis and small left pulmonary artery (LPA) are well-known problems following Norwood palliation for hypoplastic left heart syndrome (HLHS). We investigated anatomical and technical factors that may stand behind these problems. METHODS: A total of 47 consecutive patients were included in the study. All had complete magnetic resonance imaging (MRI) study pre-second-stage palliation. Measurements were taken using a first-pass 3D angiography technique after intravenous injection of an extravascular contrast agent. Factors investigated included the following: size and site of the pulmonary artery bifurcation stump in relation to the Damus-Kaye-Stansel (DKS) anastomosis, interaortic distance/ratio (neoaorta to descending aorta distance/antero-posterior dimension of the chest) (IAD/IAR), distance from the under surface of the arch and the size of native aorta and pulmonary artery. IAD/IAR were compared between two different arch reconstruction techniques. RESULTS: Stenosis occurred either centrally, at the origin of the BPA, or more distally, in the mid-LPA (posterior to DKS). There was a significant lower incidence of central BPA stenosis when the pulmonary artery stump was placed in the mid-position compared with right/left position (26 vs 67%; P = 0.011). A more bulky pulmonary artery stump was also found in those patients with central BPA stenosis (186 vs 137 mm(2)/m(2); P = 0.047). The mid-LPA consistently showed antero-posterior compression (mean cranio-caudal diameter 3.82 mm vs mean antero-posterior diameter 3.07 mm, P < 0.001). Indexed mid-LPA area was only correlated with IAD/IAR (r = 0.49 and 0.51, P < 0.001). No correlation was shown with the distance to the under surface of the arch (r = 0.14, P = 0.37), again confirming antero-posterior compression of the LPA rather than cranio-caudal. In multivariable analysis, the only predictor of indexed mid-LPA area was the IAR (P < 0.001). There was no significant difference in the IAD or IAR between the two arch reconstruction techniques [mean IAD 15.5 vs 13.5 mm (P = 0.14)]; [mean IAR 0.17 vs 0.19 (P = 0.21)]. CONCLUSIONS: Of all studied factors, IAR and the size and position of the pulmonary artery bifurcation plays the main role in LPA growth and central BPA stenosis.