Arthroscopic acromioplasty was done for a lesion of the rotator cuff in 165 patients: 100 who had stage-II impingement syndrome (no actual tear of the rotator cuff) (group 1), forty who had a partial tear (group 2), and twenty-five who had a full-thickness tear (group 3). The operation consisted of acromioplasty, resection of the coracoacromial ligament and subacromial bursa, and removal of osteophytes, when present, near the inferior aspect of the acromioclavicular joint. In the patients who had a partial or complete tear, minimum debridement of the rotator cuff also was performed. In group 1, eighty-six patients (eighty-nine shoulders) were available for review at a minimum follow-up to two years (average, 31.2 months). The preoperative ratings for pain, activities of daily living, work, and sports improved markedly in eighty-one patients postoperatively. The most common findings at operation were proliferative subacromial bursitis and an acromial protuberance. Two complications were recorded. Seven patients had a subsequent open operation on the shoulder. In group 2, the average follow-up was 28.9 months (range, twenty-four to forty-eight months). Of the forty patients, thirty-three had a major improvement in the ratings for pain, activities of daily living, work, and sports. One complication, transient palsy of the lateral femoral cutaneous nerve, was noted. Two patients who had an unsatisfactory result had a second operation: one, open acromioplasty and the other, repair of the rotator cuff. In group 3, the average follow-up was 30.8 months (range, twenty-four to fifty-five months). There were fourteen satisfactory and eleven unsatisfactory results. Of the twenty-five patients, seven later had open repair of the rotator cuff, and six had a satisfactory result from that procedure. No complications were recorded. It was concluded that arthroscopic acromioplasty is effective in the treatment of isolated stage-II impingement and partial tears of the rotator cuff. Arthroscopic treatment of complete tears produced over-all results that were inferior to those of traditional open repair. Arthroscopic subacromial decompression cannot be supported as treatment for complete tears of the rotator cuff.