The overall average of the posterior slope of the medial plateau of the tibia as determined from the lateral radiography of the knee was 10 degrees +/- 4 degrees, with a range from 0 degrees to 20 degrees. This value was independent of age and gender and was unaffected by arthrosis as long as the arcuate contours of the medial socket were not eroded and could be identified by lateral radiography. While the average value of this posterior slope was similar to that of a cadaveric study, the range was much greater. The current practice of neglecting the preoperative posterior tibial slope and arbitrarily setting the tibial component at a predetermined posterior tilt angle failed to take into account the wide variation in posterior tibial slope.