A prospective case series was undertaken to determine the incidence of os acromiale in patients presenting to an orthopedic clinic with shoulder pain during a 6-month period. Ninety-three shoulders in 88 consecutive patients were evaluated by history, physical examination, and radiographs. Radiographs included anteroposterior, scapular Y, and axillary views in all patients. Based on history and physical examination, no patient was given a primary diagnosis of os acromiale. However, when the plain films were specifically reviewed for this entity, it was present in 6 (6.82%) of the 88 patients. It was seen on the axillary view in each case. Five (83.3%) of the 6 patients had been given a diagnosis of either impingement or rotator cuff tear based on physical examination. Os acromiale is an infrequent but not rare entity that must be kept in mind when evaluating patients with shoulder pain, as failure to recognize an os acromiale may negatively influence treatment results. An axillary view should be obtained when evaluating patients with shoulder pain.