Acromioclavicular joint cyst (AJC) is a rare condition, usually occurring in the presence of a wide communication between glenohumeral and acromioclavicular joints in patients with a massive rotator cuff tear. In the presence of AJC, accurate evaluation of the status of the rotator cuff should always be made. Removal of the cyst only must be avoided because the condition tends to recur if the cuff tear is not repaired. When the tear is so large that it cannot be repaired, excision of the cyst should be associated with a resection of the lateral end of the clavicle.