The effect of sequential pneumatic leg compression on venous flow velocity was studied in 25 patients. At the usual pressures of 35-55 mm Hg, venous flow velocity was only 175% of control value, but 366% when pressures were between 90 and 100 mm Hg. The incidence of postoperative thrombosis was studied on three groups of patients. In 24 patients, receiving sequential compression prophylaxis with 90-100 mm Hg, there was one case of thrombosis, while there were three cases among the 25 patients with 35 mm Hg compression and three in the 25 receiving 5000 unit of heparin three times daily, Sequential intermittent compression with high pressures, ten compression cycles three times daily, is recommended for all patients unable to undertake active prophylactic measures. An absolute indication exists in those patients which can no be mobilised and those in whom there are additional contraindications to the use of anticoagulants. Sequential compression is no alternative to antithrombosis stockings. These must be worn from the day of hospitalisation until discharge, also during the period of pneumatic compression.