YM150 is an orally active direct FXa inhibitor in clinical development. Proof of concept of YM150 was previously obtained in a dose escalation study, demonstrating a statistically significant dose-related trend in VTE prevention in patients undergoing total hip replacement (ONYX; J Thromb Haemost2007;5:1660–5). The present study with YM150 is a double blind, parallel, multi-center dose finding study in patients undergoing elective primary hip replacement surgery, with open label enoxaparin as control. Patients were randomly treated with 5, 10, 30, 60 or 120 mg YM150 once daily orally or 40 mg enoxaparin once daily subcutaneously. Patients were treated in hospital for approximately 1–2 weeks followed by home treatment for another 4 weeks, for in total 33–38 treatment days (prolonged treatment for hip replacements according to guidelines of Am. College of Chest Physicians (Geerts et al. Chest2004:126;338–400)). The administration of YM150 started 6–10 h after surgery, while treatment with enoxaparin started 12 ± 2 h before surgery. Mandatory bilateral venography was performed after 7–10 days of treatment. Follow up visits were performed at 2 and 4 weeks during home treatment, and 2, 4, and 8 weeks after end of study treatment. The sample size was estimated to be 960 randomized patients in order to yield 110 patients per treatment group with an evaluable venogram. The primary efficacy endpoint was defined as deep vein thrombosis, and/or symptomatic VTE and/or death, and primary safety endpoint as the incidence of major bleeding during the first 7–10 days of study treatment. Secondary efficacy endpoints were proximal and distal DVT (during the first 7–10 days) and symptomatic VTE during home treatment, and follow up. Secondary safety endpoints were the incidences of major, clinically relevant non-major (CRNM) and minor bleeding during the three study periods. Independent Adjudication Committees reviewed all venograms, bleeding reports, symptomatic VTEs, and deaths unaware of treatment allocation. An independent DSMB monitored the data to ensure the safety of the patients. 1017 patients were randomized from 81 sites in 17 European countries. Preliminary data showed that DVT was detected in 149 out of 727 evaluable venograms, one patient had symptomatic VTE, and one patient died (myocardial infarction) during the hospital treatment period. This results in a total primary efficacy outcome rate of 20.7% (151/729) in the evaluable study population, with a corresponding rate of major bleeding of 0.6% (6/1017). The breakdown of the results according to treatment group will be presented.