The aim of this study was to perform biometry of the proximal extremity of the radius and to characterize the shape of the radial head. Knowledge of the size and shape of the radial head is necessary for the creation of a radial head prosthesis that is anatomically and biomechanically correct. Twenty-seven measurements, focused on the proximal extremity, were done on 96 radii. The shape of the radial head was determined by the difference between the maximum diameter and the minimum diameter. We considered the shape to be circular when the difference was less than 1 mm, and elliptical when the difference was greater than 1 mm. The shape of the radial head was compared with the neck/diaphysis angle. Fifty-seven percent of radial heads were elliptical and 43% were circular. When the head was elliptical the maximum diameter was 22 mm +/-2.9 and the minimum diameter was 20 mm +/-2.8 ( P<0.001). When the head was circular the maximum diameter was 21.2 mm +/-2.4 and the minimum diameter was 20.4 mm +/-2.4 ( P<0.14). The angle between the neck and the diaphysis varied with regard to the shape of the radial head. It was 166.75 degrees +/-3 for the circular heads and 168.62 degrees +/-3.2 for the elliptical heads ( P<0.01). The biomechanics of the circular shape and the elliptical shape are different, involving an adaptation of the angle between the neck and the radial diaphysis. This difference must be taken in consideration in the design of a radial head prosthesis.