During a four-year period, fourteen individuals (fifteen shoulders) who had been seen at the shoulder service of our institution because of pain in the shoulder had a radiographic finding of an os acromiale. On clinical examination, the pain appeared to be due to an unstable os acromiale because the patients had point tenderness over the acromion and pain on forward elevation of the shoulder. The diagnosis of an os acromiale was confirmed on radiographs, magnetic resonance images, or a bone scan. Eight patients had an associated tear of the rotator cuff. The os acromiale was located in the pre-acromion in one shoulder, the meso-acromion in eleven shoulders, and the meta-acromion in three shoulders. At the operation, the anterior aspect of the acromion was found to be unstable in all shoulders.Eleven patients (twelve shoulders) had open reduction of the os acromiale and insertion of an autogenous iliac-crest bone graft. Of those patients, four (five shoulders) had open reduction and internal fixation with a tension-band procedure with use of pins and wires. Only one of those shoulders had a solid osseous union, and the other four shoulders had a non-union that was due to a disruption of the fixation. The remaining seven patients (seven shoulders) had open reduction and internal fixation with use of cannulated screws and a tension-band construct; a solid osseous union was achieved in all but one of them. One patient had excision of the pre-acromion, which relieved the pain. Two patients who had had failed open reduction and internal fixation had excision of a grossly unstable os acromiale in the meso-acromion; both patients had pain and weakness after this procedure.Of the twelve shoulders that had open reduction and bone-grafting, seven had union of the os acromiale; the average time to radiographic and clinical union was nine weeks (range, seven to twenty weeks). We concluded that, although it is rare, symptomatic unstable os acromiale does occur and can be effectively treated with use of autogenous bone-grafting and internal fixation with a rigid tension-band construct and cannulated screws.