Roentgen stereophotogrammetric analysis has been used to study ankle/foot kinematics in eight healthy volunteers. All the joints between the tibia and the first metatarsal as well as the talo-calcaneal and tibio-fibular joints were analysed in input plantar flexion/dorsiflexion and pronation/supination of the foot as well as internal/external rotation of the leg. The findings included the following: INDIVIDUAL JOINTS 1. The joint axis of the talo-crural joint varied with varying kinds of input motion. Substantial amounts of rotation occurred about axes close to the vertical; this occurred particularly when the input motion was in the internal rotation part of the arc of leg rotation and in pro-/supination of the foot. 2. The total amount of rotation in the talo-calcaneal joint was small in internal rotation of the leg and in pronation of the foot compared to external rotation of the leg and supination of the foot. 3. The talo-navicular joint showed a limited ball-and-socket joint pattern in all subjects. The total amounts of rotation were larger than in the talo-calcaneal joint in all subjects. Plantar flexion axes were more transverse than the axes seen in other kinds of input motion. 4. The talo-calcaneal and talo-navicular joint axes were seldom parallel, indicating that these joints do not necessarily behave as a simple hinge. JOINT INTERACTION Joint interaction varied in different qualities of input motion. Plantar flexion induced rotation in the talo-crural joint, and to some extent in the joints of the arch. Dorsiflexion mainly induced talo-crural joint motion. Pronation/supination induced motion in all joints. The distal joints of the arch displayed more rotation in pronation than in supination, while the talo-calcaneal joint showed less motion in pronation than in supination. Internal leg rotation induced little rotation in the joints of the ankle/foot complex. External rotation induced external rotation, dorsiflexion, and supination in the talo-navicular and talo-calcaneal joints. The distal joints of the arch displayed compensatory plantar flexion and pronation. TRANSFERRAL OF ROTATION The ankle/foot complex showed ability to transform leg rotation into pro-/supination and vice versa. This function was most pronounced in external leg rotation. MOTIONS OF THE FIBULA The fibula showed consistent lateral and posterior translation from input plantar flexion to dorsiflexion of the foot.