We evaluated whether near-infrared spectroscopy (NIRS) measurement from the flank correlates with renal vein saturation in children undergoing cardiac catheterization. Thirty-seven patients <18 years of age were studied. A NIRS sensor was placed on the flank, and venous oxygen saturations were measured from the renal vein and the inferior vena cava (IVC). There was a strong correlation between flank NIRS values (rSO(2)) and renal vein saturation (r = 0.821, p = 0.002) and IVC saturation (r = 0.638, p = 0.004) in children weighing ≤ 10 kg. In children weighing > 10 kg, there was no correlation between rSO(2) and renal vein saturation (r = 0.158, p = 0.57) or IVC saturation (r = -0.107, p = 0.67). Regional tissue oxygenation as measured by flank NIRS correlates well with both renal vein and IVC oxygen saturations in children weighing <10 kg undergoing cardiac catheterization, but not in larger children.