Traumatic posterior dislocation of the shoulder is frequently missed because of its rarity and the absence of characteristic symptoms. Several signs should be emphasised: an overlap of the humeral head and glenoid rim in a true anteroposterior view and the light-bulb sign in the anteroposterior view. To make an accurate and early diagnosis, use of multidirectional radiographs combined with computed tomography is recommended. Closed reduction was successfully performed under general anaesthesia using the DePalma method with slight modification--the lever principle--by pushing the medial side of the upper arm laterally to adduct the shoulder as far as possible. The dynamics of the lever principle make it a safer and more effective method of achieving a closed reduction of a posterior dislocation of the shoulder than the conventional method of solely pushing the humeral head anteriorly, especially in patients with locking of the glenohumeral joint and impression fractures.