Attempts to separate congenital radioulnar synostosis and restore forearm rotation had been disappointing until a new mobilization technique was developed by Kanaya and colleagues using free vascularized fascio-fat graft interposition. This technique provides a functional rotation arc, but postoperative forearm motion is difficult to evaluate given the inaccuracies in determining the range of motion and rotational axis using conventional x-rays or computed tomography. This study represents an attempt to analyze in vivo three-dimensional motion of the forearm with radioulnar synostosis treated by Kanaya's operation using a markerless bone registration technique. Six patients with seven postoperative forearms (six with congenital and one with posttraumatic radioulnar synostosis) underwent 3D computed tomography with the forearm in neutral, fully pronated, and fully supinated positions. Range of motion according to this method was 30 ± 18°, significantly smaller than the 82 ± 29° from manual examination. Improvements in range of motion were significantly greater in cases without dislocation of the radial head (46 ± 13°) than in cases with dislocation (19 ± 10°). Dislocation of the radial head was also significantly correlated with an abnormal axis of rotation of the forearm.