The effect of citrated stored blood on coagulation was studied initially in a pilot study where 25 patients with acute severe gastrointestinal haemorrhage had their whole blood coagulation measured using the Biobridge Impedance Clotting Time (ICT). This demonstrated that there is a hypercoagulable response to haemorrhage which was partially reversed by blood transfusion. Similar changes were noted in Kaolin Cephalin Clotting Times (KCCT). A further 50 patients were then randomized to receive, during the 24 h after admission, either at least 2 units of blood or no blood transfusion unless the haemoglobin fell below 8 g/dl or they were shocked. In the transfused group nine patients re-bled compared with only one in the non-transfused group (P less than 0.01, chi 2 with Yates' correction). Early blood transfusion appears to reverse the hypercoagulable response to haemorrhage thereby encouraging re-bleeding and hence the need for an operation.