Hyperkalemia is considered a medical emergency as it can result in severe disturbances in cardiac rhythm and death. Although many causes of hyperkalemia exist, exogenous red blood cell transfusions are being recognized as the primary perioperative etiology. The authors report a case of severe intraoperative hyperkalemia associated with the use of allogeneic blood products (packed red blood cells), during a surgical mission to a developing country. The patient was undergoing repeat mitral valve replacement with cardiopulmonary bypass (CPB) and developed significant hyperkalemia with a serum potassium value of 9.9 mEq/L. Successful intraoperative therapies were instituted with a gradual reduction in the serum potassium value to 4.8 mEq, which allowed the patient to be weaned from CPB. The authors review the etiology of hyperkalemia in children including its relationship with allogeneic red blood cell transfusions and treatment modalities including specific therapies which can be instituted during CPB.