Background: There is increased early postoperative mortality after elective joint replacement surgery. However, the magnitude and duration of the increased mortality are uncertain. Methods: Data on total knee and total hip replacement from the comprehensive national registries in Australia and Norway were assessed. Only patients between fifty and eighty years of age with osteoarthritis were included. Overall, the study included 81,856 patients with a total knee replacement and 106,254 patients with a total hip replacement. Smoothed intensity curves were calculated to show the change in mortality for the early postoperative period, whereas the effects of risk factors were studied with use of the nonparametric additive Aalen model. Results: We found that early postoperative mortality was increased for the first twenty-six postoperative days (95% confidence interval, twenty-two to forty-one days). The excess mortality, compared with a baseline mortality (calculated as the average mortality from Day 100 to Day 200), for these twenty-six days was estimated to be 0.12% (95% confidence interval, 0.11% to 0.14%). The most important risk factors for excessive early postoperative mortality were male sex and high age (more than seventy years of age). Conclusions: There is an increased, but low, early postoperative mortality following lower extremity joint replacement surgery. The excess mortality persists, but steadily decreases, for approximately the first twenty-six postoperative days. Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.