The incidence of postoperative deep vein thrombosis (PDVT) after aortic surgery and lower limb revascularisation has not been assessed by a large prospective study. In a prospective randomised trial the effect of a low-molecular-weight heparin fragment, Enoxaparin (ENX) 4200 anti factor Xa IU once daily was compared to that of unfractionated heparin (UFH) 7500 IU twice daily. Two hundred and thirty-three consecutive patients were classified into three groups, aortic or aortoiliac and aneurysmectomy (n = 75), aorto-femoral bypass for atherosclerotic disease (n = 71), and femoropopliteal or femorodistal bypass (n = 87). Patients were analysed for development of deep vein thrombosis by Duplex scanning and, if positive, by venography between the seventh and tenth postoperative day. PDVT was present in 10 patients in the ENX group and in four patients in the UFH group (8.2 and 3.6% respectively, NS). The incidence of PDVT was 8% after aortic or aortoiliac aneurysmectomy, 7% after aortofemoral revascularisation, and 3.4% after femoropopliteal or femorodistal bypass. The overall incidence of PDVT after aortic surgery was 7.5% (95% CI 5.4-9.7). There was no pulmonary embolism. Intra-operative blood loss and postoperative bleeding events did not differ significantly between the ENX and UFH groups. After 1 month follow-up, no clinical event or death could be related to PDVT or pulmonary embolism. In conclusion, in vascular surgery ENX is as safe and effective in the prevention of PDVT as is UFH.