Total knee arthroplasty (TKA) and total hip arthroplasty (THA) are associated with high rates of venous thromboembolism (VTE). Anticoagulants, such as warfarin, are commonly used to prevent VTE in such patients. The practice and effectiveness of warfarin in real world populations is not well documented. 595 TKA and THA patients treated with warfarin were followed by two anticoagulation clinics in the Michigan Anticoagulation Quality Improvement Initiative. Length of prescribed anticoagulation, percent time in therapeutic range (%TTR) and time to first therapeutic INR were calculated for each surgical group. For THA, all 300 patients received at least ten days with a 28-day median length of anticoagulation therapy. For TKA, all 295 patients received at least 10 days with a 28-day median length of anticoagulation therapy. For THA patients, time to first therapeutic INR was on average 12.0 ± 8.0 days with a mean %TTR of 36.6 ± 26.8% for goal INR 2.0-3.0. For THA patients, 39 (13%) never reached target INR. For TKA patients, time to first therapeutic INR was on average 12.8 ± 10.3 days with a mean %TTR of 36.0 ± 28.3% for goal INR 2.0-3.0. For TKA patients, 44 (14.9%) never reached target INR. Many orthopaedic surgeons who use warfarin for post-arthroplasty VTE prophylaxis do so in accordance with national guidelines. The time to first therapeutic INR is strikingly long and %TTR markedly low for these patients, raising questions about the efficacy of warfarin therapy in the first 1-2 post-operative weeks. Further studies to investigate the best target INR for warfarin prophylaxis, as well as the composite rates of VTE and clinically relevant bleeding from treatment with warfarin, LMWH and newer anticoagulants are needed.