Posterior dislocation of the shoulder is rare. It may follow trauma, electroconvulsive treatment or an epileptic attack. Correct diagnosis may be delayed, as there is no obvious deformity in the shoulder region and the arm is held in a natural sling position. AP X-ray of the shoulder shows only a few minor signs. A high index of suspicion, proper physical examination, and oblique X-ray views lead to correct diagnosis. Closed reduction is possible if the dislocation is of less than 3 weeks duration. Open reduction is indicated in young patients, and in older patients when there is disability.