Acromioclavicular joint cyst is a extremely rare condition, usually occuring in the presence of a wide communication between glenohumeral and acromioclavicular joint in patients with a full thickness rotator cuff tear. Removal of cyst only is reported with high recurrence rate. There was no case previously reported in Korea. We report a case of recurred acromioclavicular joint cyst with a massive rotator cuff tear after simple excision, which was treated by excision of cyst, acromioplasty and resection of the lateral end of the clavicle.