Forty-five patients with brain tumours were studied for evidence of any haemostatic abnormalities in the preoperative and intraoperative period. An abnormality was found in 44 of the patients in the preoperative period and in all the patients during the operation. One patient developed acute disseminated intravascular coagulation. A change in the haemostatic abnormality from the preoperative to the intraoperative period was demonstrated in 9 of the patients. The histological nature of the tumour did not influence the haemostatic derangement. A shortened euglobulin lysis time, prolonged thrombin time, increased fibrin degradation products and abnormal fibrinogen levels were the common isolated abnormalities. When considered together, chronic disseminated intravascular coagulation (DIC) with or without fibrinolysis and fibrinolysis with or without DIC were the commonest abnormalities. Although some degree of haemostatic derangement is found in a high proportion of patients with brain tumours, clinically relevant abnormalities are rare.