['This study evaluated the aerobic capacity, exercise capacity, and arterial oxygen saturation (O(2)Sat) in children before and after transcatheter Fontan fenestration closure.', 'Observational study comparing exercise parameters and hemodynamics before and after transcatheter fenestration closure in Fontan patients.', 'Working capacity, exercise duration, oxygen consumption (VO(2)), and arterial O(2)Sat were evaluated during aerobic exercise.', 'Twenty patients (mean age 11.4 years) underwent standardized exercise testing before and after fenestration closure. Twelve patients underwent cycle ergometry testing (mean age 14.8 years) (group 1), and eight younger patients (mean age 6.4 years) underwent Bruce treadmill testing (group 2). The same exercise protocol was used in each patient before and after fenestration closure (interval between tests: 118 +/- 142 days). Immediately following fenestration closure at cardiac catheterization, cardiac index decreased (3.0 to 2.1 L/minute/m(2)) and Fontan pressure increased (11 +/- 2 to 12 +/- 2 mm Hg) with an increased arterial saturation (92 to 96%) (P < .001). The total group demonstrated no significant change in pre- and postclosure maximal heart rates (164 +/- 21 and 169 +/- 19 bpm). Rest and exercise O(2)Sat increased (89 and 82 to 95 and 92%) (P < .0001). Exercise duration increased (7.7 +/- 1.9 to 9.2 +/- 2.4 minutes) (P < .0005). Maximal VO(2), indexed maximal VO(2), and total working capacity in kilopond-meters (kpm) increased (1.2 +/- 0.5, 27 +/- 7 and 2466 +/- 1012 to 1.3 +/- 0.4 L/minute, 31 +/- 9 mL/kg/minute and 2869 +/- 1051 kpm, respectively) (P < .005).'] Does exercise capacity improve after transcatheter closure of the Fontan fenestration in children?