Background: Hydroxyethyl starch (HES) solutions are often used for maintaining intravascular volume and improving microperfusion, while a large amount of HES can cause adverse effects on coagulation. As the indications for clopidogrel expand, an increasing number of patients undergoing off-pump coronary artery bypass surgery (OPCAB) are also undergoing dual antiplatelet therapy (DAPT), with its higher risk of bleeding complications. The aim of the present study was to determine whether a moderate dose of 6% HES 130/0.4 significantly increases perioperative blood loss in patients with continued DAPT within 5 days of OPCAB. Methods and Results: Patients who received clopidogrel and aspirin within 5 days of OPCAB were randomly allocated to receive HES 130/0.4 (≤30ml/kg) followed by crystalloid infusion (HES group, n=53), or crystalloid only (crystalloid group, n=53) perioperatively. The amount of perioperative blood loss (sum of bleeding during the intraoperative and postoperative 24-h period), transfusion requirements, modified thromboelastography and coagulation variables, hemodynamic parameters, and fluid balance were recorded. Perioperative blood loss and coagulation profiles were similar between the groups, but the postoperative hemoglobin level was higher in the crystalloid group. Conclusions: Up to 30ml·kg^-1·day^-1 of 6% HES 130/0.4 did not increase the perioperative blood loss compared to crystalloid in patients with recent exposure to DAPT undergoing OPCAB. HES 130/0.4 caused a similar degree and duration of coagulation impairment as observed when only crystalloid was given. (Circ J 2011; 75: 2397-2402)