The effects of hydroxyethyl starch on blood coagulation were investigated in 20 patients undergoing surgery to determine whether its use places recipients at risk of haemorrhage or thrombosis. The partial thromboplastin times are significantly prolonged; factor VIII activities and fibrinogen levels are decreased. After infusion of hydroxyethyl starch, no significant differences were detected in platelet count or prothrombin time. A decreased platelet aggregation was also found after the infusion of hydroxyethyl starch. According to our results, hydroxyethyl starch can cause haemorrhagic problems in patients when administered as a colloidal volume-expanding agent.