Background: Although whole blood viscosity (WBV) had close relation with acute coronary syndrome (ACS), there have been few studies for the serial changes of WBV in ACS patients. The aim of the study was to investigate the serial change of WBV in ACS patients. Methods: Consecutive 24 patients who underwent diagnostic coronary angiography were enrolled (63.3짹9.2, 12 males) and were divided to two groups [group I = non-ACS (n=8), group II = ACS (n=16)]. We excluded the patients who had a history of recent TIMI major bleeding, major operation within 1-month, cardiogenic shock, uncontrolled severe hypertension, contraindication to the anti-platelet agents, coronary artery spasm, a previous history of myocardial infarction, or severe hematological diseases including anemia (hemoglobin < 10.0 g/dL). Patients under 18 or over 80-year old were also excluded. The blood samples were obtained from each patient just before diagnostic coronary angiography and at 1-month later. All patients were administered dual anti-platelet therapy during follow up. Systolic and diastolic WBV was measured at a shear rate of 300 and 1 s-1 by a scanning capillary viscometer (BVD-PRO1, Bio-Visco Inc., South Korea). Results: The systolic and diastolic WBVs between acute and 1-month follow up were not significantly different in both groups. (Table) Conclusions: Even though there were numerical decreases of diastolic WBVs in ACS during 1-month follow up, further large studies are needed to clarify the present results.