The direct and indirect effects of injury are to stimulate coagulation and fibrinolysis, almost immediately, in direct proportion to the severity of the trauma, and with effects which are both local and remote from the site of damage. A series of bloodchanges follows in quick succession and may lead to a phase of apparent hypocoagul-ability and reduced fibrinolysis, although even at this stage there is evidence of continued stimulation of both systems. It is important to examine the mechanism of these changes in order to understand, and to be prepared to counter, the sudden haemostatic failure or thromboembolism which may result. The accelerated blood clotting after injury is easy to see. William Hewson, describing the killing of sheep in 1772, observed that'the blood which issued last coagulated first'. Stimulation of fibrinolysis is less readily appreciated but the unusually fluid character of the blood at necropsy in cases of violent death was commented on by Morgagni (1769) and by John Hunter (1794). The frequent succession of a'positive'phase of hypercoagulability and a'negative'phase of hypocoagulability was described by Wooldridge in a series of papers beginning in 1886. To examine the process further it will be neces-sary to outline current concepts of blood coagula-tion and fibrinolysis before describing the changes observed, discussing their mechanism, and their possible significance.