Acromioclavicular joint (ACJ) cyst is an uncommonly documented condition of the shoulder, first described by Craig. 5 According to Hiller's classification, 13 there are 2 types of cysts, based on the various etiologies that generate them. Type I cysts are caused by degenerative changes of the ACJ due to injury, metabolic/infectious disease, or excessive use and are therefore restricted to the joint, with an intact rotator cuff. Gradually, these changes can inflame the synovium, leading to the overproduction of fluid and cyst formation superficial to the ACJ. Type II cysts are associated with rotator cuff tears, partial or complete, and subsequent cuff tear arthropathy, including superior migration of the humeral head and irritation of the inferior ACJ capsule. Synovial fluid from the glenohumeral joint (GHJ) escapes through the subacromial/subdeltoid bursa (Geyser sign) into the ACJ, with the deteriorated capsule acting like a 1-way valve that prevents the fluid from returning, leading to the formation of a cyst. 6 We report a massive cyst over the right ACJ in an 80-yearold man, followed by a short literature review. PubMed database search was carried out using the terms acromioclavicular joint cyst, acromioclavicular joint ganglion cyst, ACJ cyst, and ACJ ganglion cyst. No date criteria were placed. All articles reporting the cyst's size and its underlying pathology, as well as the applied treatment methods and the recurrence rate, were included. The inclusion criteria were (1) full-text studies and (2) English language or official English translation provided. The initial research query provided 166 records. Eleven were excluded as duplicates, and 79 were irrelevant by reviewing the title. In addition, 2 cases describing a juxtaarticular myxoma and chondromyxoid fibroma, respectively, were excluded. From the remaining 76 reports, 10 were rejected by reviewing their abstracts, because of apparent irrelevancy. We tried to retrieve the full text of the remaining 66 articles. For 10 of them, the whole text could not be obtained; therefore, they were excluded. No official English translation was provided for 8 articles, which were also not included. A total number of 48 cases were thoroughly reviewed. Six were rejected because of irrelevant data and 6 because of a low level of evidence. A total of 34 studies finally entered the review, reporting 57 cases of ACJ cysts (Table I). A right-handed 80-year-old man was admitted to the hospital with a mass located above his right ACJ. His medical history was free, with the exception of pulmonary echinococcosis, diagnosed 40 years ago. The patient, a retired manual laborer, denied any previous shoulder trauma or any prior surgeries on the affected side, but complained of chronic right shoulder discomfort and mild pain during daily life activities, because of a gradually increasing shoulder lump. On physical examination, the mass measured 6.5 cm in diameter, 8 cm in length, 5 cm in Institutional review board approval was not required for this case report. This patient provid...