(1) To analyse the imaging appearances of nine patients with acromioclavicular joint cysts presenting as shoulder masses for tumor staging with operative, histopathological and joint aspiration findings. Retrospective review of imaging and correlation with clinical, operative and surgical notes. Images were reviewed by two musculoskeletal radiologists by consensus. Nine patients who presented clinically with a shoulder mass were evaluated by radiographs (n=9), ultrasound (n=1), conventional arthrography (n=3), MRI (n=6; with direct MR arthrography n=2, indirect MR arthrography n=4). All patients had a focal mass superior to the AC joint, with a size ranging from 1.5 cm to 6 cm and a mean of 3.27 cm. Correlation was available with surgery (n=7), histopathology (n=2) and cyst aspiration (n=2). Two patients were managed conservatively. Geyser sign was positive in all three arthrograms. All MRIs revealed extensive rotator cuff tears with a column of fluid extending from the glenohumeral joint through the rotator cuff tear into the acromioclavicular joint and acromioclavicular cyst. Chondrocalcinosis was seen in the acromioclavicular joint cyst (n=2) and in the glenohumeral joint (n=1). Aspirate in two patients contained calcium pyrophosphate dihydrate crystals. Acromioclavicular joint cysts may present as a tumor mass. They are associated with extensive rotator cuff tears and there is usually communication of the cyst with the joint space. This feature excludes a diagnosis of tumor. AC joint cysts may be associated with calcium pyrophosphate dihydrate deposition disease.