Previous studies of elbow kinematics produced conflicting results because of technical limitations. Kinematic analyses of eight normal elbows (four in cadavera and four in living subjects) using true lateral roentgenograms made by a special technique showed that flexion occurs about a single axis. This axis passes through the centers of the arcs outlined by the bottom of the trochlear sulcus and the periphery of the capitellum. Motion of the joint surfaces is of a sliding type except at the extremes of flexion and extension where rolling motion occurs. The carrying angle of the forearm remains constant as the elbow flexes. Based on these findings, total elbow prostheses ideally should be uniaxial, with the axis of flexion of the prosthesis the same as the axis of the normal joint.