BACKGROUND The fibular fracture requires an anatomical reduction. When a malunion occurs, it can lead to a valgus deformity with an opening of the ankle mortise. CASE REPORT This case deals with a 23-year-old patient with pain and limited movement in the right ankle, caused by a fracture healed in an incorrect position, with shortening of the fibula and progressive displacement of the valgus, after surgery 12 months earlier for an ankle fracture. The patient underwent a corrective procedure consisting of extemporaneous lengthening of the fibula, with interposition of autologous bone graft and fixation using a compression plate. Six months after surgery, the patient did not present pain or limited movement, and was able to return to his habitual sporting activity. CONCLUSIONS Fibular-lengthening osteotomy is a procedure indicated for patients with malunion fracture of the distal fibula, with shortening and progressive valgus deformity. This surgery allows the restoration of the joint surface, reduces stress on the cartilage and prevents the development of arthropathy of the ankle.