The occurrence of supradiaphragmatic decompressing venous collateral channels following construction of a bidirectional cavopulmonary connection or completion of the Fontan operation resulting in abnormal systemic hypoxemia has been infrequently described. In addition, the incidence and predictors of these channels have not been well delineated, especially in those patients without formation of such structures preoperatively. I evaluated, retrospectively, 40 patients who had undergone either construction of a bidirectional cavopulmonary shunt or completion of the Fontan operation, and who had complete pre and postoperative hemodynamic and angiographic data. Of the patients, 17 (43%) had developed a total of 21 decompressing venous collateral channels, of which 7 (18%) were considered to be hemodynamically significant requiring transcatheter coil occlusion. Of all variables examined, seven patients with significant decompressing collaterals had a greater transpulmonary gradient at follow-up catheterization (8 + /− 2 vs 5 + /− 2 mmHg, p= .01) and lower systemic saturations at routine clinical follow-up visits (82 + /− 5 vs 89 + /− 5 mmHg, p= .007) in comparison to the 33 others. When not evident preoperatively, decompressing venous collateral channels develop in a significant number of patients following conversion to Fontan physiology. If sufficiently large, they may produce lower than expected systemic saturations for the observed cardiac physiology. The larger decompressing channels are more likely to occur when a greater transpulmonary gradient exists postoperatively, which may require cardiac catheterization and transcatheter coil occlusion.