A chronic cyst overlying the acromioclavicular joint was managed in four patients, between July 1988 and September 1991. All patients had had previous unsuccessful aspiration and excision of the cyst with recurrence. Each cyst was associated with a chronic, massive defect of the rotator cuff; superior migration of the humeral head; and degenerative osteoarthrosis of the glenohumeral joint. All patients had complained of pain and limitation of motion (mean forward elevation, 95 degrees; mean external rotation, 20 degrees; and mean internal rotation, to the spinous process of the second lumbar vertebra). All procedures consisted of a large-humeral-head hemiarthroplasty, with no operative treatment directed at the cyst or the acromioclavicular joint. At an average of twenty-seven months (range, fifteen to thirty-six months) after the operation, the patients were all pain-free and had not had a recurrence of the cyst. The average postoperative range of motion was 130 degrees of forward elevation, 30 degrees of external rotation, and internal rotation to the spinous process of the first lumbar vertebra.