Objectives: In recent decades, most centers have implemented fenestrated Fontan-Kreutzer surgery in patients with congenital heart diseases and univentricular physiology. This has shown to improve postoperative course and shorten hospital stay. However, fenestration can remain open for years, leading to systemic desaturation due to persistent right-to-left shunt, with a relative increase in the risk of thromboembolic events and brain abscesses. Transcatheter closure of the fenestration in patients who do not experience spontaneous closure is anticipated to improve hemodynamic parameters. Currently, there is no universal consensus on parameters to determine who should undergo closure. The purpose of this study is to describe the parameters used in our experience and subsequent outcomes. Methods and results: Retrospective study between December/2015 and March/2024, including patients with Fontan-Kreutzer surgery who underwent cardiac catheterization with fenestration occlusion test to evaluate the possibility of closure with endovascular device. A total of 19 patients were taken to the catheterization laboratory, 10 were male (52.6%), with a mean age at Fontan of 6.7 years (CI: 4-8.5) and mean age at closure attempt of 11.4 years (CI: 6-16). The most common underlying diagnosis was pulmonary atresia (n=7, 36.8%), followed by tricuspid atresia (n=6, 31.6%). 89.5% (n=17) presented with cyanosis, the main reason for considering closure. 11 (57.9%) patients had successful closure, 5 (26.3%) were not considered closure candidates, and 2 (10.5%) had fenestration in the process of closure at the time of catheterization. In the successful closure group, an average system pressure of 12.63 mmHg (CI: 10-15) was found compared to 16.2 mmHg (CI: 13-19.5) in those considered non-closure candidates. Closure of the fenestration was more likely in patients with normal end-diastolic pressure of the single ventricle, without ventricular dysfunction or no more than moderate atrioventricular valve insufficiency. The most used device for closure was the Amplatzer ASD (n=8, 72.3%). Within the group of patients in whom fenestration was closed, only 1 (5.3%) presented with protein-losing enteropathy; none of the patients in whom fenestration was not closed presented with subsequent complications. Conclusion: Closure of the fenestration in patients undergoing Fontan-Kreutzer surgery is shown to be an effective and safe procedure in those individuals with favorable hemodynamics during the occlusion test. Determining factors influencing the decision to close or keep the fenestration open include Fontan pressure and variations in cardiac output during the procedure. These aspects influenced the fenestration closure, after which a significant improvement in oxygen saturation and symptoms was observed, without procedure-related complications. Performing a fenestration occlusion test is essential for accurate decision-making in this clinical context. Further studies with larger cohorts are needed to establish definitive criteria for fenestration closure in this patient population.