The recently published American Association of Orthopedic Surgeons (AAOS) guidelines for the prevention of venous thromboembolism (VTE) in patients undergoing hip or knee surgery conflict with long-established and widely used American College of Chest Physicians (ACCP) guidelines. Both guidelines accepted that the most important goal of thromboprophylaxis in patients undergoing hip or knee replacement is to prevent pulmonary embolism (PE). The ACCP guidelines included asymptomatic (and symptomatic) deep vein thrombosis (DVT) detected by venography as a measure of the efficacy of thromboprophylaxis, whereas the AAOS rejected DVT as a valid outcome because the panelists considered the link between DVT and PE to be unproven. The AAOS position is inconsistent with evidence from imaging studies linking DVT with PE and from clinical studies demonstrating a parallel reduction of DVT and PE when antithrombotic agents are compared with placebo or untreated controls. The AAOS panel ignored the randomized data demonstrating that thromboprophylaxis reduces both DVT and PE, and many of their recommendations are based on expert opinion and lack a scientific basis. We recommend the ACCP guidelines because the methodology is explicit and rigorous and the treatment recommendations reflect all of the evidence from the randomized trials. Adoption of the ACCP guideline will ensure that patients undergoing hip and knee arthroplasty receive the best available therapies for prevention of VTE and reduce disability and death due to this common and potentially preventable condition.