Six months after undergoing a Fontan procedure for tricuspid valve atresia, a 7-year-old child experienced thrombotic occlusion of the Hancock conduit coincident with an acute gastroenterologic illness that produced sudden, severe dehydration. Recovery from this complication was possible only by virtue of pulmonary blood flow through a Glenn anastomosis, which had been constructed at 1 year of age and which was left in place at the time of the Fontan procedure. At subsequent reoperation, the Hancock conduit, which was completely occluded with organized thrombus, was replaced with another Hancock conduit, and the patient is now doing well 10 months after this last operation. The relatively nonpulsatile flow characteristic of blood passing through the prosthetic conduit after the Fontan operation predisposes the patient to the possibility of the potentially lethal complication, previously unreported, of thrombotic occlusion of the conduit, and care must be taken to avoid severe dehydration in such patients.