Children with chronic cyanotic heart disease often develop systemic-to-pulmonary artery collateral vessels that can be deleterious at the time of a Fontan procedure because of excessive pulmonary blood flow with resultant ventricular volume overload. We therefore occlude all significant collateral arteries during preoperative cardiac catheterization. From June 1993 to September 2001, 137 children ranging from 1.5 to 18.3 years old (median, 2.4 years), underwent a fenestrated lateral tunnel Fontan procedure. Of these, 130 (95%) had a previous bidirectional Glenn anastomosis, including 43 (31%) with a Norwood procedure. Preoperatively, 52 children (38%) required occlusion of collateral vessels. Two of five perioperative deaths (operative survival, 96%) resulted from excessive pulmonary blood flow; one from unrecognized collateral arteries and one from uncontrollable collateral arteries. Postoperatively, 29 children (22%) required coil occlusion of collateral vessels for elevated pulmonary artery pressures, heart failure, or prolonged chest tube drainage. At follow-up of 1.5 months to 8.3 years (mean, 4.1 years), there have been four late deaths (two from pneumonia, two secondary to heart failure); nine patients underwent cardiac transplantation for refractory heart failure. Ten of 11 patients with ventricular failure required occlusion of significant collateral vessels postoperatively. Hemodynamically significant collateral arteries are common in Fontan candidates. Aggressive control can result in good early and medium-term survival. After the Fontan operation, the presence of significant collateral vessels may be a marker for eventual cardiac failure; 11 of the 29 patients who required postoperative coil placement went on to transplantation or died of heart failure. Copyright © 2002 by W.B. Saunders Company