An outline has been given of the major abnormalities of coagulation which can occur secondary to diseases in previously normal individuals. First, the disorders due to deficiency of the vitamin K-dependent clotting factors are described. Vitamin K deficiency can occur in the newborn, or at later stages in life when there is intestinal malabsorption. The malabsorption disorders, such as coeliac disease, together with major abdominal surgery or prolonged use of broad-spectrum antibiotics can give rise to vitamin K deficiency. Additionally, in obstructive jaundice the lack of secretion of bile salts into the upper intestine causes vitamin K malabsorption. The use of oral anticoagulants is associated with haemorrhage in a small proportion of patients. These patients usually have an excessively prolonged prothrombin time, due to overdosage with anticoagulants, but occasionally haemorrhage can occur from a localized bleeding site, such as a duodenal ulcer, in patients under good anticoagulant control. The large number of drugs which can interact with anticoagulants are listed, from which it can be seen that careful monitoring of all patients on oral anticoagulants must be carried out. The haemostatic defects associated with liver disease are then tabulated. In this situation abnormalities may be due to deficient synthesis of coagulation factors in hepatocellular failure, by failure of vitamin K absorption, and also by disseminated intravascular coagulation (DIC). DIC occurs in hepatocellular failure, because the liver cells are normally responsible for clearing activated products of the coagulation and fibrinolytic enzyme systems. The presence of clinical haemorrhage and haemostatic breakdown in hepatic disease usually indicates a serious prognosis, but appropriate replacement therapy is indicated in this situation. Disseminated intravascular coagulation embraces a large number of clinical haemorrhagic syndromes, where intravascular activation of the coagulation system takes place accompanied by compensatory fibrinolytic activity. DIC can be initiated by intravascular release of procoagulant substances, such as tissue thromboplastin, or by damage to vascular endothelium and platelets. The main clinical conditions associated with DIC comprise the severe infections and septicaemias, obstetric accidents, shock and trauma, neoplasia and snake-bite envenoming. In all instances, the pathophysiological disorder of haemostasis is managed by treating the underlying disease.(ABSTRACT TRUNCATED AT 400 WORDS)