Introduction: Osteotomy is a recognised treatment to correct fracture malunion, requiring skill and planning; historically based on orthogonal X-rays on which it is difficult to gauge rotation. This is especially important in the forearm, whhere minimal displacwement can greatly affect function. 3D osteotomy cutting jigs can aid planning and performance to improve patient outcomes. We present the first reported case in New Zealand of such a system used in a provincial hospital to treat forearm malunion in a four-year-old boy, with two-year follow-up. Methods: Patient/whānau consent was obtained for use of the system and write-up of this article. A four-year-old with forearm malunion underwent three-dimensional planning for osteotomy with bilateral forearm CT imaging, referencing defined anatomic landmarks and standard alignment parameters. Computer-assisted planning for osteotomy resulted in patient-specific cutting and drilling guides being designed and 3D printed for intra-operative use. Results: Though guides for both bone osteotomies were designed, intra-operatively, after the radius had been completed, the forearm was taken through full range of motion with no mechanical block felt. Therefore, the ulna was not instrumented. Immediate post-operative films showed correction of the radius malunion by 18’ medially and 18’ anteriorly compared to the predicted 22.7 and 18.8’ respectively. The planned rotation of 32’ was seen intra-operatively. There were no complications after the index or removal operations, scar was satisfactory and radiographic union was seen at three months. Forearm movement was symmetrical to his other side after 3 months, preserved after plate removal. Conclusion: Three-dimensional planning and patient-specific cutting jigs are valuable tools for understanding and correcting complex forearm malunions. They are a useful salvage tool that can improve the accuracy and risks of osteotomy. Optimal outcomes are achieved when advanced planning technologies are used to augment, rather than replace, clinical decision-making.