PURPOSE The risk of deep vein thrombosis (DVT) in patients undergoing arthroscopic knee surgery is not well known. The purpose of this study was to determine the incidence of DVT, to demonstrate the efficacy of a perioperative and postoperative prophylaxis against thromboembolism with use of low--molecular weight heparin (LMWH), and to show the safety and feasibility of LMWH administration. TYPE OF STUDY Prospective, single-blind, randomized clinical trial. METHODS There were 218 consecutive outpatients scheduled for ambulatory arthroscopic knee surgery eligible. Of these, 130 patients were randomized to a treatment group with LMWH (dalteparin: 2,500 IU less-than-or-equal70 kg and 5,000 IU >70 kg, started perioperatively and given once daily for 4 weeks; n = 66) and a control group (n = 64) with no prophylaxis. To detect DVT, all patients underwent bilateral compression ultrasonography before and 12 and 31 days after surgery. RESULTS Among the 130 patients studied, thromboembolism was significantly lower in the treatment than in the control group: 1 of 66 (1.5%) versus 10 of 64 (15.6%); 95% confidence interval, 7.8% to 26.8%; P =.004. Eighty percent of DVT occurred within the first 14 postoperative days. No severe side effects of LMWH were observed. Only 5% of patients refused continued subcutaneous LMWH injections. CONCLUSIONS In patients undergoing ambulatory arthroscopic knee surgery without antithrombotic prophylaxis, the risk of DVT is high. Perioperative and postoperative prophylaxis with dalteparin is an effective and safe means of reducing this risk.