A procedure was introduced for determining the thickness of the articular cartilage of the patella. The results revealed that the cratilage was thicker at the site of contact between the patella and the femur than in other parts. CT and X-ray images were used to investigate movement of the patella from full extension to full flexion of the knee joint. As the flexion increased, the patella prorated, i, e., the site of contact with the epicondyle and condyle of the femur shifted from the lateral to the medial facet of the articular face, as deduced from the decrease in the bicondylo-patellar angle. Vertically, the patella moved from the epicondyle to the condyle level, but the distance between the apex of the patella and the tuberosity of the tibia remained almost unchanged. Serial slices (5 mm thick) of a plaster replica of the patella cut at right angles to the central ridge showed a change in the angle between the two facets of the articular face from the proximal to the distal side. The same results were obtained with CT slices. These findings suggest that adequate pressure stimulates development of the cartilage and that CT is a good method for studying movement of the patella.