The kinematic technique of instant center analysis of the knee is described. Determination of the instant center or centrode permits identification of the type of motion of the joint surfaces. Twenty-five normal knees were studied and the centrodes were located for the range of motion between full extension and 90 degrees of flexion. In all cases, the contacting surfaces rotated about centrodes which produced motions whose velocities at the joint surface were tangent to the surface. Thirty knees with internal derangements were also analyzed. These joints had instant center pathways which were abnormal in some portion of the range of motion of each joint. The centrode abnormalities were associated with surface velocities which tended to force the joint surfaces together or apart and to increase surface friction. Damage to articular cartilage was present at the contact areas corresponding to the knee joint positions which had abnormal centrode positions. A mechanism of development of degenerative joint disease secondary to a remote traumatic internal derangement is described.