Three patients developed protein-losing enteropathy caused by intra-atrial obstruction of the systemic venous return after Mustard's operation. The enteropathy resolved in one case after reoperation and in the others after balloon dilatation of the stenosed caval pathways. Protein-losing enteropathy may occur as a complication of Mustard's operation. Balloon dilatation of the obstructed baffle is an effective alternative to reoperation.