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Objective: To compare enoxaparin and dextran 70 for the prophylaxis of venographically diagnosed deep vein thrombosis (DVT) after gastrointestinal operations., Design: Part 1: randomised double blind trial; Part 2: single blind study with historical controls., Setting: Eight Norwegian hospitals., Subjects: 381 Patients undergoing elective gastrointestinal operations., Interventions: Part 1 (n = 329): enoxaparin 20 mg subcutaneously starting two hours before operation and continuing until the patient was fully mobilised or had had 10 injections and a placebo infusion of 0.9% sodium chloride, or dextran 70,500 ml at the start of the operation, on the evening of operation, and on the first, third, and fifth postoperative days and placebo subcutaneous injections. Part 2 (n = 52): enoxaparin 40 mg in the same regimen as part 1 (compared with 39 historical controls). Venograms 4-6 days post-operatively., Main Outcome Measure: Venographically confirmed DVT., Results: Part 1: Because of the high overall incidence of DVT an interim analysis was done which showed 33/101 DVT (33%) among high risk patients in the enoxaparin 20 mg group and 33/107 (31%) in the dextran 70 group. The corresponding figures for patients at medium risk were 2/27 (7%) for enoxaparin 20 mg and 5/27 (19%) for dextran 70 (95% confidence interval (CI) for the difference--11.9 to 9.8). Part 2: the dose of enoxaparin was therefore increased to 40 mg and prophylaxis restricted to patients with cancer. There were 6/49 DVT (12%), which was compared with a random sample from the dextran 70 group from part 1 (historical controls) in which the incidence was 15/39 (38%, 95% CI of the difference 4.0 to 8.4). There were no pulmonary emboli, only 4 thrombi were above the knee and there were 4, 1 and 3 clinical DVT in the 20 mg and 40 mg enoxaparin, and dextran 70 groups, respectively., Conclusions: Enoxaparin 20 mg and dextran 70 are effective prophylaxis for patients at medium risk, but enoxaparin 40 mg is required for those at high risk.