Ten femoral heads, three of them grossly pathological, have been examined to determine their shape, employing seven types of testing. We found the sagittal section of the normal femoral head to be remarkably circular, with very close concentricity of the surface of the articular cartilage and the underlying bone. The surface of the cartilage seen in the sagittal section was even more truly circular than the surface of the underlying bone. Comparing the circularity of sections in three planes the greatest deviation from true circularity was found in the section seen in the frontal plane. We found no position of the hip joint in which the femoral head protrudes from the acetabulum more than in any other, nor have our observations revealed any inherent stability of the hip joint attributable to peculiarities in the shape of the opposed surface, enabling these to lock one into the other. Study of the circularity of the eburnated parts of arthritic femoral heads suggests that the process which keeps the normal femoral head spherical is well-known in production engineering, and is of such a nature that the attainment of high degrees of sphericity is not at all surprising.