Type II Monteggia lesion equivalents produced by plastic deformation of the ulna are rare. Radial head fractures in skeletally immature patients are also uncommon. We report a late presentation of a Type II Monteggia equivalent injury with a fracture of the radial head and neck and plastic deformation of the ulna in an 11-year-old boy. The radial head was located on the initial injury radiographs and subsequently dislocated in a posterior direction. The radial head fracture was misdiagnosed as a coronoid fracture at presentation. The plastic deformation of the ulna was diagnosed several weeks after the injury when the dislocation was first noted. Magnetic resonance imaging was used to establish the diagnosis of a radial head fracture at the same time. The patient was treated successfully with an open reduction and internal fixation of the radial head combined with a dorsal closing-wedge ulnar osteotomy.