OBJECTIVE To evaluate the magnetic resonance (MR) imaging findings of os acromiale in rotator cuff disease. METHODS Retrospective analysis was performed of 10 shoulder MR studies of 10 patients older than 25 years with os acromiale. The authors subdivided these patients into three types on the basis of seven possible configurations. RESULTS Eight of the 10 patients with os acromiale showed failure of fusion between the mesoacromion and meta-acromion (type A), one between the preacromion and mesoacromion (type B), and one between the meta-acromion and basiacromion (type C). Osteophytic lipping was seen at the margins of the acromial gap in nine cases. The os acromiale could be detected on the oblique sagittal image, oblique coronal image, or both, and in each case, there was a lesion in the supraspinatus tendon: tendinitis in four patients and tendon tear in six. CONCLUSIONS Os acromiale can be an important cause of rotator cuff impingement and can be detected with routine MR imaging of the shoulder.