Malunion of recurrent fractures in Osteogenesis Imperfecta (OI) patients causes limb length discrepancy and malrotation. These cause added difficulty for OI patients to ambulate. Lengthening with distraction osteogenesis using an external fixator in OI patients is challenging. Acute lengthening with autologous bone graft is a known method in a normal bone but not a known procedure in OI patients. We present two clinic cases of adolescent OI patients with limb length discrepancy and externally rotated lower limb that underwent acute lengthening and rotational correction using a locked intramedullary nail and ipsilateral autologous iliac bone graft. Both patients obtained union and improvement of ambulatory capability without recurrence of fracture within five years of follow-up. Acute lengthening by 2cm and rotational correction with intramedullary nail improved the gait efficiency in the OI patients. Harvesting large amounts of the tricortical iliac bone graft, followed by controlled weight-bearing is a safe procedure.