A 13-year-old boy weighing 35 Kg underwent surgery to correct pulmonary valve atresia by total caval-pulmonary anastomosis (Fontan procedure). During surgery and recovery, no adverse events were observed. On the third day after surgery, the patient developed signs of low cardiac output, renal failure, and right ventricular insufficiency. In spite of an antithrombotic treatment protocol with sodium heparin (10 IU/Kg/h), venous return was found to be impeded by 2 thrombi in the inferior vena cava. Thrombolysis with urokinase was ineffective, and surgical thrombectomy was only partially successful. Finally, a high dose of intravenous heparin was prescribed (25 IU/Kg/h) and the thrombi were dissolved. The importance of prophylactic antithromboembolic therapy following corrective surgery for congenital heart disease must be emphasized. Ultrasound monitoring of these patients, even in the absence of clinical signs, has proven effective for early detection of the thrombotic events described in this report.