BACKGROUND: To avoid the risks associated with transfusion of homologous blood products, artificial colloid solutions represent an alternative for intra-operative blood loss replacement. However, synthetic colloids have been implicated as a cause of coagulopathy when administered in large quantities. We investigated the effect of Hydroxyethyl starch (HES) on blood coagulation in vitro under thromboelastography (TEG). METHODS: Whole blood was withdrawn from fifteen volunteers undergoing peripheral surgery who had no history of coagulation defect. Whole blood was diluted with HES to 25, 50 and 75 vol% concentrations, and the changes in coagulation status were analysed using TEG and were compared with those of an undiluted control specimen obtained concurrently from the same patients. RESULTS: Hemodilution with HES solution at 50 vol% concentration decreased the MA and alpha angle values (P < 0.05), but the R and K values remained unchanged. In case of profound hemodilution at a 75 vol% concentration, the values of MA and alpha angle were severely decreased (P < 0.05) and the values of R and K were severely increased (P < 0.05). CONCLUSIONS:There were many reports that moderate hemodilution with crystalloids increased coagulability, but hemodilutions up to 50 vol% concentration with HES solution did not significantly change blood coagulability. Significant hypocoagulability occurred at a 75 vol% hemodilution with HES.