We studied the blood sugar levels after hypothermic cardiopulmonary bypass (CPB) in diabetics and non-diabetics. Twenty eight patients were divided to the following four groups by the preoperative value of hemoglobin-A1c (HbA1c) and medication; (1) HbA1c < 6.0 and given oral anti-diabetic medication, (2) HbA1c > 6.0 and given oral anti-diabetic medication, (3) HbA1c > 6.0 and given insulin injection, and (4) non-diabetics. Both in diabetics and non-diabetics, the blood sugar levels were higher than 300 mg.dl-1 during cardiopulmonary bypass. In group (1) and (4), the blood sugar levels went down lower than 250 mg.dl-1 at 60 minutes after CPB, without any treatment. In group (2) and (3), the blood sugar levels after 60 minutes of CPB were higher than 250 mg.dl-1, and insulin 8 units were given. After 30 minutes of insulin injection, the blood sugar levels of group (2) were lower than 250 mg.dl-1, but those in group (3), the blood sugar levels were still higher than 250 mg.dl-1, and another 8 units of insulin were given. The diabetic patients whose HbA1c levels were higher than 6.0 and given anti-diabetic medication before operation, need insulin after CPB.