Background:The traumatic dislocation of the radial head in children is commonly treated by closed reduction. Sometimes, however, this strategy of treatment may not be effective due to the location of soft tissues in the radio-shoulder joint. The literature presents a few cases of the irreducible radial head dislocation with ulnar plastic deformation. Because it is a relatively rare condition, such a traumatic dislocation can be easily missed. Neglected injuries can lead to unwanted complications and unpredictable surgical outcomes.Case presentation: This study presents a relatively rare case of traumatic radial head dislocation with ulnar plastic deformation in a 3-year-old child, which was successfully treated by open reduction. The examined case did not require osteotomy and ligamentous reconstruction. The initial attempt of closed reduction failed due to annular ligament interposition, which has been detected on MRI. After 3 months of treatment, the range of motion of the operated arm gradually improved. At the 6-month follow-up, the Mayo elbow-performance score indicated an excellent treatment outcome. Conclusions:The delayed treatment of radial head dislocation with ulnar plastic deformation can hinder the supination and pronation of the forearm, resulting in elbow/forearm deformity. The earlier this condition is detected, the easier it will be to treat it and the better the treatment outcome will be. The examined case of irreversible traumatic dislocation, successfully treated by open reduction, may help to treat radial head dislocation better.