Surgical site infection (SSI) is a serious operative complication that may be associated with any surgical procedures. It increases morbidity and mortality after cardiac surgery. Plasma concentration of prophylactic antibiotic, routinely cefazolin, is altered by effects of cardiopulmonary bypass. This study is conducted to measure the plasma concentrations of cefazolin to determine its adequacy in pediatric patients undergoing elective cardiac surgery with cardiopulmonary bypass including its correlation with the incidence of postoperative SSI.