In a retrospective and prospective study, 303 patients with fractures of the ankle joint treated surgically with postoperative immobilization in a plaster cast were analysed. After the introduction of thromboembolism prophylaxis with heparin for all outpatients immobilized by plaster casts the incidence of thromboembolic complications decreased significantly, from 4.5% to zero. The prophylaxis was well accepted, about 90% of the subcutaneous injections being administered by the patients themselves or family members. No serious complications were seen. Therefore, this effective prophylaxis of thromboembolism should be practised in all outpatients with a plaster cast on the lower limb and an elevated risk of thromboembolism. A simple check-up to identify patients at risk is suggested.