Rationale:Elbow injury in children by improper treatment or a delay of more than 3 weeks could lead to old unreduced Monteggia fracture, which are difficult to manage. Conservative or normal surgical methods usually fail.Patient concerns:Herein, we present a 6-year-old boy with sustaining injury approximately 1 month to his left elbow. Activity in his elbow was restricted, and his ability to extend his wrist and fingers was impaired.Diagnoses:Type III Monteggia elbow fracture-dislocation consisting of radial head dislocation and malunion of the ulna associated with posterior interosseous nerve palsy were confirmed, which requiring surgical treatment.Interventions:A closed reduction was performed with hyperplastic scar tissues erased and the radial head relocated.Outcomes:Follow-up 4 months later showed satisfactory recovery of function.Lessons:Forearm fractures in children may be misjudged, and that early anatomical reduction rather than conservative treatment may be required.