Progressive Prominent Swelling Over the Acromioclavicular JointAn 81-year-old man (no relevant medical history, no medications) reported a progressive, mobile swellingover the acromioclavicular (AC) joint, present for several months. Sonographic examination confirmed theliquid nature of the mass. Three aspirations of the content by the patient’s family physician before eventual referral to the hospital were successful only in the very short term: just a few hours later, theswelling was as large as before. The patient suffered no pain during normal daily activity, but was bothered by the appearance of the lesion and above all by the difficulty he experienced in getting dressedand in wearing his rucksack. Magnetic resonance imaging showed advanced osteoarthritis of the AC jointwith a prominent AC joint cyst and the characteristic geyser sign. Because of the clinical symptoms thepatient was treated with arthroscopic resection of the AC joint and open resection of the AC joint cyst. Sixmonths later the skin and operation scar showed no signs of inflammation. No recurrence was reported.