This study of normal patello-femoral biomechanics defines some functional specifications which may be useful in future total knee prosthesis design. Serial lateral X-rays of 15 fresh knees and their patellar mechanisms at several flexion angles provided definition of the direction of the resolved patello-femoral forces. Assuming that the patella acts as a frictionless pulley, the magnitude of the patellofemoral forces during several activities was calculated using data from Morrison (1970) and Smidt (1973). It ranged between 421 and 3420 newtons for the various activities and for isometric exercise. 9 methylene blue contact print technique was used to measure the bearing areas. These data indicate that between 13 and 38 per cent of the patellar surface bears joint loadings. Patello-femoral contact stresses were calculated to range from 1.28 to 12.6 N/mm². A 696 newton man climbing stairs would, for example, generate a patello-femoral force of 1754 newtons and would experience patello-femoral contact stresses between 3.73 and 6.87 N/mm². Stress values were equal to or in excess of anticipated tibial-femoral stresses. The high patello-femoral load values, the small bearing surfaces, and the consequent significant stress magnitudes indicate the need for caution in development of a patello-femoral joint prosthetic replacement.