In previous randomised clinical trials of thromboprophylaxis after total hip replacement, low-molecular-weight heparin has been given for an arbitrary 7 to 14 days. The risk factors are mainly perioperative and it is possible that a shorter course may be adequate. We assessed the safety and effectiveness of a three-day course. We assessed 156 primary THR patients after randomisation to either a control group or to receive enoxaparin at 12 hours preoperatively and 12 and 36 hours postoperatively. Thrombosis was diagnosed by routine venography. Haemorrhagic side-effects were assessed by measurement of blood loss, and soft-tissue side-effects by descriptive scores for wound discharge and bruising of the leg. The prevalence of calf thrombosis was 15.4% in the enoxaparin group and 32.1% in the control group (p = 0.01); the prevalence of proximal thrombosis was 15.4% and 17.9% respectively (not significant). There was no difference in haemorrhagic side-effects or wound discharge, but there was more bruising in the enoxaparin group.