An instrumented tibial prosthesis was developed to measure forces in vivo after total tibial arthroplasty. This prosthesis was implanted in a 67-kg, 80-year-old man. The prosthesis measured forces at the 4 quadrants of the tibial tray. Tibial forces were measured postoperatively during rehabilitation, rising from a chair, standing, walking, and climbing stairs. By the sixth postoperative week, the peak tibial forces during walking averaged 2.2 times body weight (BW). Stair climbing increased from 1.9 times BW on day 6 to 2.5 times BW at 6 weeks. This represents the first direct in vivo measurement of tibial forces, which should lead to refined surgical techniques and enhanced prosthetic designs. Technical design improvements will enhance function, quality of life, and longevity of total knee arthroplasty.