The problem of post-operative thromboembolism is re-examined and the progress made in the prevention of venous stasis and hypercoagulability reviewed. The high incidence of asymptomatic phlebitic phenomena is noted and personal experience of the use of low doses of Calcium heparin in 415 patients undergoing various types of operation is reported. Eight patients had chronic duodenal ulcers. A notable reduction in the incidence of post-operative thromboembolism was confirmed. The value of this preventive method is thus demonstrated, as is the usefulness of analysing haemostatic balance in patients with one or more risk factors for thromboembolism in order to "personalize" the dosage and duration of treatment with Calcium heparin.