We had an opportunity to evaluate a child who developed fever approximately two to three weeks after the open heart surgery for tetralogy of Fallot. His peripheral blood smear showed rings and various stages of Plasmodium vivax. The patient had received packed red blood cells during the surgery and postoperative care, one unit of which was later proved sero-positive for malaria. The possibility of malaria should be included in the differential diagnosis of the patients with unexplained fever after multiple blood product transfusions for the open heart surgery.