Synthetic colloids have been reported to cause haemorrhagic complications. The effects of perioperative volume replacement with 4% gelatin (n=20), 6% low-molecular weight (LMW) hydroxyethyl starch (HES) (Mw: 70 000 dalton; HES 70/0.5; n=20) and 6% medium-molecular weight (MMW) HES (Mw: 200 000 dalton; HES 20010.5; n=20) on haemostasis were assessed in patients undergoing major abdominal surgery. Volume was administered to keep central venous pressure (CVP) between 10 and 14 mm Hg. Conventional global coagulation tests, molecular markers of coagulation, and platelet function (using a platelet function analyser (PFA100(TM)) with ADP as inductor) were monitored prior to surgery (T0), at the end of surgery (T1), 4 h after the end of surgery (T2), and on the morning of the first postoperative day (T3). Significantly more gelatin (2900 (SD 320) ml) than HES 200 (2150 (312) ml) was given during the study period, Bleeding and the use of allogeneic blood-blood products were similar in all groups. Markers of thrombin generation (F1+2), of thrombin neutralization (TAT III complex), and of fibrin formation and its degradation (D-dimer) increased significantly during and after surgery without showing significant group differences. Factor VIII and von Willebrand factor (vWF) also increased in all groups beyond the normal range, showing the significantly highest increase in the gelatin-treated group (VIII: from 173 (36) to 266 (33) U dl(-1); VWF: from 164(33) to 238 (31) U dl(-1)). Platelet function remained within the normal range and without group differences throughout the study period. We can conclude that all three solutions can be used safely in patients undergoing major abdominal surgery with regard to the haemostatic process.