OBJECTIVES: This study aimed to evaluate the relationship between pulmonary artery size and venovenous collaterals during staged single ventricle reconstruction. METHODS: Patients who underwent staged Fontan palliation between 2003 and 2023 were reviewed. The relationship between the pulmonary artery index and the development of venovenous collaterals was determined. Furthermore, the impact of pulmonary artery index and venovenous collaterals on in-hospital morbidities after the Fontan procedure was evaluated. RESULTS: A total of 377 patients were included. Median age at bidirectional cavopulmonary shunt and total cavopulmonary connection were 4.2 (3.3-6.2) months and 2.1 (1.7-2.6) years, respectively. Venovenous collaterals were observed in 51 (13.5%) of the patients. Patients who developed venovenous collaterals showed higher pulmonary artery pressure (p = 0.024), higher transpulmonary gradient (p = 0.042), lower pulmonary artery index (p = 0.016), and lower right pulmonary artery index (p = 0.011) at the time of bidirectional cavopulmonary shunt, compared to those without. However, the pulmonary artery index was similar in patients with and without venovenous collaterals at the time of total cavopulmonary connection. Higher transpulmonary gradient (p = 0.007) and lower pulmonary artery symmetry index (p = 0.032) at bidirectional cavopulmonary shunt were identified as independent risks for developing venovenous collaterals. The existence of venovenous collaterals did not influence the postoperative course after total cavopulmonary connection. Notably, pulmonary artery symmetry index at bidirectional cavopulmonary shunt was identified as an independent risk for prolonged pleural effusion (p = 0.018) and for chylothorax (p = 0.021). CONCLUSIONS: A small and unbalanced pulmonary artery at bidirectional cavopulmonary shunt is associated with the postoperative development of venovenous collaterals.