A 53-year-old man was urgently hospitalized with chronic renal failure, congestive heart failure, pulmonary edema, and pneumonia. He received respiratory support and dialysis after hospitalization in the intensive care unit. Coronary arteriography revealed an old myocardial infarction and unstable angina (triple vessel disease). Surgery was planned. However, after dialysis under heparin administration, clot formation was noted in the dialyzer. Serological tests confirmed the presence of antibodies to heparin-platelet factor 4 complex ; accordingly, heparin-induced thrombocytopenia (HIT) was diagnosed. Coronary artery bypass surgery should preferably be performed early in the case of coronary artery disease. However, surgery during the acute phase of HIT when antibodies to heparin-platelet factor 4 complex (HIT antibodies) are present is associated with a very high risk of developing thromboembolism. There is no criterion regarding the optimal timing for surgery when HIT antibodies are present. Therefore, clinicians are often confused about this. In cases where the platelet count, D-dimer level, fibrinogen degradation product (FDP) level, and fibrinogen level improve, thrombin production due to HIT antibodies is thought to decrease. We considered that the improvement in these values suggests that the number of HIT antibodies decreases and thus HIT antibody activity would be reduced. We evaluated the platelet count, D-dimer level, FDP level, and fibrinogen level over time and accordingly determined the optimal timing for surgery. In the present case, argatroban administration was started after HIT developed, and the platelet counts increased gradually ; the D-dimer and FDP levels decreased, whereas there were no significant changes in the fibrinogen levels. Although HIT antibodies were still present, we performed off-pump coronary artery bypass grafting under the administration of argatroban when the platelet count, D-dimer, and FDP values improved. The patency of coronary bypass grafts was confirmed postoperatively ; the patient did not develop thromboembolism during the perioperative period and was discharged without complications. When HIT antibodies are present, an improvement in platelet count, D-dimer, and FDP values is thought to be useful in determining the optimal timing of surgery.