In this report, the conservative treatment for the complete dislocation of acromioclavicular joint by plaster spica is discussed.Sixteen patients were treated, 15 being men and one woman. The average age was 32 years (range, 19-43). We applied plaster spica with 90 degrees arm abduction to immobilize the scapula and reduce the distal end of the displaced clavicle using elastic band. Thus managed, the articular surface of the acromioclavicular joint contacts each other closely. The alignment of reduced acromioclavicular joint must be confirmed by X-ray for 4 to 6 weeks. 13 cases completed treatment in this manner but 3 cases gave up treatment halfway owing to severe pain or failure of reduction. Follow-up periods averaged 1 year and 7 months.Pain relief was generaly satisfactory but anatomical reduction was obtained in only 6 of the 13 completed cases. The relations between reduction and the shape of the acromioclavicular joint were discussed in this series according to Urist's classification. 8 of the 13 cases were type II (Vertical) or type V (Incongruent & No contact) and the other 5 were type I (Overriding) or type IV (Incongruent & Overriding). 6 cases which gained anatomical reduction in this series were all classified into type II and type V.Anatomical reduction seemed to be important for the improvement of function and the relief of symptoms. The results were not so good in older patients because of residual contracture.