To assess superior vena caval (SVC) obstruction after the Mustard operation for transposition of the great arteries, we performed two-dimensional contrast echocardiography (2-D contrast echo) in 18 patients, ages 1-9 years. Sterile saline was injected into a peripheral scalp or arm vein while the junction of the inferior vena cava (IVC) and the systemic venous atrium (SVA) was imaged from the subcostal long-axis plane. The results of 2-D contrast echo were compared with those obtained at cardiac catheterization. In nine patients, contrast passed from the SVC to the SVA and the IVC remained free of contrast echoes. At catheterization, these patients had no SVC obstruction by angiography and minor SVC-SVA mean pressure differences (0-4 mm Hg). In five patients, contrast passed from the SVC into the SVA. Within a few cardiac cycles, contrast from azygos-IVC collateral vessels flowed in the IVC toward the SVA. At catheterization, these patients had partial SVC obstruction by angiography and SVC-SVA mean pressure differences of 9-13 mm Hg. In four patients, the SVA was filled only by contrast arriving from the IVC by way of azygos-IVC collateral vessels. At catheterization, these patients had complete SVC obstruction and SVC-SVA mean pressure differences of 14-20 mm Hg. Two-dimensional contrast echocardiography is a simple, accurate, noninvasive method for detecting SVC obstruction after Mustard's operation and allows differentiation of complete from partial SVC obstruction.