Ischemia/reperfusion (I/R) injury is an important adverse effect of cardiopulmonary bypass (CPB) in infants undergoing cardiac surgery. We performed a multicenter, randomized, placebo-controlled, double-blinded trial of the leukocyte-endothelial cell adhesion inhibitor Cylexin in young infants to determine if it reduces I/R injury following hypothermic CPB. Entry criteria included age at surgery 1-45 days, birth weight > 2.3 kg, and planned repair or palliation of congenital heart defects with CPB. We excluded patients with specified antecedent events or conditions, including lung or kidney disease, seizures, necrotizing enterocolitis, infection, or other serious noncardiac morbidity. Randomization was stratified by study center and cardiac diagnosis. Cylexin was administered continuously from sternotomy until 24 hours post CPB. Centers followed their usual operative and postoperative care practices. From December 1997-March 1999, we enrolled 230 infants, 155 for 2-ventricle repairs (Group 1: D-TGA=90, VSD=16, TOF/truncus=22, TAPVR=9, VSD with aortic arch obstruction=18) and 75 for stage 1 palliation (Group 2: single ventricle with aortic arch obstruction). Of those enrolled, 117 were assigned to Cylexin (Group 1=80, Group 2=47) and 113 to placebo (Group 1=75, Group 2=38). Pre- and intraoperative variables were comparable between treatment groups. Early (30 day) mortality for Cylexin versus placebo patients in Group 1 was 0% versus 3.8% (p=0.25) and for Group 2 was 10.8% versus 28.9% (p=0.08). In both risk groups, treatment with Cylexin did not significantly improve other early postoperative outcomes or decrease the occurrence of adverse events. Cylexin did not significantly improve early mortality or postoperative recovery in Group 1 patients. Despite a small sample size, early mortality in Group 2 Cylexin-treated patients tended to be lower, suggesting the need for future trials of agents that could reduce I/R injury in high-risk infants.