The postoperative thrombosis of prosthetic grafts may be due to many factors: technical errors, poor run-off, prosthetic material, graft length, trauma by subcutaneous tunnellization or repeated microtrauma across the joint areas, evolution of atherosclerotic lesions, emorheological changes. In 50 patients submitted to surgical arterial repair of the lower limbs, we have employed 8,000 I.U. AXa/daily of LMW-Heparin, injected subcutaneously for 6 months after the operations to prevent immediate and late thrombosis. During the follow-up, one patient died, four stopped any treatment and in two the medication was changed. Hence our results are based on 43 cases: 10 patients operated upon by aorto-femoral bypass, 19 femoro-popliteal and 14 extra-anatomical procedures. During the follow-up all the patients were investigated by ultrasounds (pressure measurement, waveform analysis and duplex scanning echotomography); moreover 13/43 (30%) were studied by angioscintigraphy and 11/43 (25.5%) by a conventional or digital subtraction angiography. Thrombosis of the grafts at one year term occurred in none aorto-femoral, in one femoro-popliteal (5.2%) and in one extra-anatomical bypass (7.1%). This figure compares favourably with the results obtained in our experience in the patients treated by a variety of drugs. In such group the incidence of occlusion is 3.9% in aorto-femoral, 9.3% in femoro-popliteal and 11% in extra-anatomical grafts. Those results emphasize the possibility to improve the patency of the grafts in the arterial repair of the lower limbs by LMW-Heparin overall in femoro-popliteal and extra-anatomical areas.