We are often faced with the question as to the optimum duration of secondary prophylaxis with oral anticoagulants after an episode of venous thromboembolism. Theoretically if we know the recurrence rate, the case-fatality, the effectiveness of oral anticoagulant therapy, and the rate of fatal haemorrhage on treatment, we can calculate whether being on or off treatment is safest. Using these data and considering only the risk of death we would treat idiopathic deep vein thrombosis for six months. For those with DVT associated with a transient risk factor it would be reasonable to stop treatment after 3 months in those over 50 years old and we should certainly stop after 3 months in those over 70 years old. There are data to suggest that pulmonary embolism may have a higher case-fatality than deep vein thrombosis if there is a recurrence. If these data were accepted most patients with idiopathic pulmonary embolism would get long-term treatment. We can use these models to modify our assessment if other factors such as antiphospholipid antibodies or cancer are present.