The U.S. Hip and Knee Registry is a voluntary national registry of outcome data for hip and knee surgery. In total, 469 orthopedic surgeons from 325 hospitals reported data on 7677 patients who underwent total hip arthroplasty (THA) and 11,461 patients who underwent total knee arthroplasty (TKA). Data on patients enrolled between 1996 and 2000 indicate that 40% of THA patients and 42% of TKA patients received prophylaxis for a period of 8 to 21 days, including the in-hospital period. Prolonged prophylaxis for more than 21 days was used in 49% of THA patients and 44% of TKA patients. Moreover, year on year, data indicate that the percentage of patients who receive prophylaxis for longer than 21 days is gradually increasing. In 2000, 53% of THA patients and 47% of TKA patients received prophylaxis for longer than 21 days.