Preoperative factors associated with serum haptoglobin levels in 35 patients before open heart surgery were evaluated, and the relationship between the incidence of hemoglobinuria with cardiopulmonary bypass (CPB) and preoperative haptoglobin level were analyzed. Inflammation increased haptoglobin levels, but the level of 2-2 type of haptoglobin were lower than those of other types of haptoglobin. In valvular disease, 5 of 6 patients with valvular sclerosis of the aortic valve had reduced haptoglobin levels and two patients had anhaptoglobinemia. Hypohaptoglobinemia seemed to be observed more frequently in aortic valvular disease than in mitral valvular disease. During CPB, serum hemoglobin increased at 0.36mg/dl/min, but haptoglobin levels at the initiation of CPB decreased to less than 30% of preoperative levels, therefore, for similar periods of CPB, the incidence of hemoglobinemia in patients with preoperative hypohaptoglobinemia was higher than in patients without preoperative hypohaptoglobinemia. Preoperative haptoglobin determination is required in candidates for open heart surgery, and haptoglobin administration is recommended in patients with hypohaptoglobinemia.