18 patients with severe hypertriglyceridaemia (mean fasting plasma triglyceride 5.7 mmol/l) had significantly higher concentrations of plasma fibrinogen and clotting factor Xc than did a normolipidaemic comparison group. Fibrinolytic activity was significantly lower in the hyperlipidaemic patients. Six months of treatment (diet or diet and clofibrate) lowered the patients mean plasma triglyceride to 3.1 mmol/l and caused a fall of clotting factors VIIc and Xc and a significant rise in fibrinolytic activity. None of these variables changed in the comparison group. Raised fibrinogen and factor VIIc concentrations are risk factors for cardiovascular mortality, and raised factor Xc and lowered fibrinolytic activity have both been found in groups at high risk of ischaemic heart-disease. Despite the fact that in population studies triglycerides do not consistently appear to be an independent risk factor for ischaemic heart disease, these data suggest that a pronounced increase in triglycerides warrants energetic therapy because it may be associated with a "hypercoagulable state".