Hydroxyethyl starch (HES) is a modified natural polymer of amylopectin, derived of potato starch or corn starch. A variety of different HES solutions exist worldwide which differ greatly in their pharmacological and pharmacocinetic properties. Tha aim of our study was to investigate the influence on coagulation and platelet function of two 6%low molecular weight hydroxyethyl starch solutions made of potato and corn starch, compared to a controll group with Ringer solution. 37 patients undergoing elective orthopedic surgery were randomly assigned to one of three groups. Hemoglobin, hematokrit, fibrinogen, von-Willebrand-factor, Factor VIIIc, Risocetin-Cofactor, prothrombin time, activated partial thromboplastin time (aPTT), and platelet maximum aggregation were measured before infusion and 30 min and 240 min after infusion. For observation of rheological changes plasma viscosity und colloid osmotic pressure were determined. The reduction of prothrombin time and the decrease of hemoglobin, hematocrit and fibrinogen were due to hemodilution. The prolongation of aPTT and the decrease of factor VIIIc, von-Willebrand-factor and ristocetin-cofactor were similar in each group. The measurement of primary hemostasis also showed an initial effect of hemodilution, but an statistically significant enhancement of coagulation 240 min after infusion, dued to an acute phase reaction and perioperative stress. Despite of physiochemical differences of the two HES solutions there were no clinically apperent effects on coagulation and platelet function compared to hemodilution with Ringer solution. Both preparations of hydroxyethylstarch can be used equally in the clinical and perioperative setting.