The effect of surgical correction of impaired forearm rotation on associated body movement patterns was studied prospectively by comparison of preoperative and postoperative three-dimensional video analysis of the upper extremity and trunk in eight male and two female patients with hemiplegic cerebral palsy (CP; mean age 16y 2mo [SD 4y 11mo]; range 11–27y). A customized parameter, ‘extrinsic forearm rotation’, was used to quantify associated movements supplementing forearm rotation. After surgical correction of the pronation deformity, active forearm supination during a functional reaching task had improved by a mean of 37° in combination with significantly decreased extrinsic forearm rotation by a mean of 13°. In addition, an average loss of 16° of active pronation in combination with increased extrinsic forearm rotation (mean 8°) was observed. On the basis of these results we conclude that successful surgical correction of a pronation deformity in patients with CP directly affects related movement patterns of the upper extremity and trunk.