A patient with moderately severe heat stroke was studied with particular attention to changes in haemostasis. Activation of fibrinolysis, a probable failure of hepatic synthesis of coagulation factors and severe thrombocytopenia were observed, but there was no definite evidence for disseminated intravascular coagulation (DIC). It is suggested, however, that in heat stroke of greater severity the more extensive tissue breakdown may lead to DIC. If DIC is superimposed on the above changes in haemostasis it is likely to be both sudden and overwhelming.