In the study presented the effects of in vitro hemodilution with HES on coagulation are compared with the effects of in vivo hemodilution using thrombelastography (TEG). The in vivo hemodilution was performed by the i. v. infusion of 1000 ml 6 % solution of hydroxyethyl starch HES (2 formulations with HES 130/0.4 and one formulation with HES 200/0.5) in healthy volunteers during 30 min. The in vitro hemodilution was performed with blood samples taken from the volunteers before the infusion was started. These samples were diluted with HES-solution until the same hemoglobin concentration measured at the end of the infusion was attained. The in vivo TEG-parameters remained in the reference range of the method, however all in vitro TEG-parameters are out of the range of normal values. The isolated interpretation of the in vitro data shows an impairment of blood coagulation. The shortening of the reaction time as an indicator for the initiation of blood clotting points to activated coagulation by in vivo hemodilution with HES, whereas in vitro the prolongation of the reaction time is indicative for retardation of clotting. The evaluation of the TEG-parameters and of other clotting parameters determined prior to the beginning of the infusion, at the end of the infusion and four hours after termination of the infusion of HES 130/0.4 and of HES 200/0.5 to healthy volunteers show alterations of blood coagulation parameters caused mainly by dilutional effects. The more pronounced alterations found in vitro cannot be interpreted as impairment of haemostasis in vivo. On the other hand, the likewise haemodilution in vitro causes an impairment of coagulation. Therefore, the effects of in vitro hemodilution with HES on coagulation differ qualitatively and quantitatively from the effects of in vivo hemodilution.