We have compared the effects of progressive (30% and 60%) in vitro haemodilution with hydroxyethyl starch (HES), gelatin (GEL) and albumin (ALB) with haemodilution using 0.9% saline in 96 patients by thrombelastography. Haemodilution with HES, GEL and ALB significantly (P < 0.05) compromised coagulation time (k), angle alpha and maximal amplitude (MA), with HES having the most negative effect at 30% and 60% haemodilution (P < 0.05). Haemodilution with saline significantly affected all variables of blood coagulation and clot lysis measured by thrombelastography, resulting in an increased coagulability at 30% haemodilution. To specifically assess the intrinsic effect of plasma expander molecules on blood coagulation and clot lysis, we analysed the difference between saline diluted blood (same degree of haemodilution) and plasma expander diluted blood. Prolongation of reaction time (r) was found for HES at 30% and 60% haemodilution and for ALB at 60% haemodilution and an increase in clot lysis by HES, GEL and ALB became evident. We conclude that HES, GEL and ALB compromised blood coagulation, while the maximum effect was found with HES.