We performed a prospective, randomized study to determine whether arthroscopic subacromial decompression changes the outcome of rotator cuff repair. We performed a power analysis to ensure statistical validity. Patients scheduled for arthroscopic rotator cuff repair were randomized to cuff repair with arthroscopic subacromial decompression (group 1) or without it (group 2). All other aspects of the surgical and postsurgical treatment were identical. We included patients with full-thickness tears limited to the supraspinatus tendon and a type 2 acromion. We excluded patients with prior surgery, those with larger tears involving two or more tendons, those with a type 1 or 3 acromion, those with workers' compensation claims, and those who had concomitant procedures (labral repair, acromioclavicular joint resection) There were 47 patients in group 1 and 46 in group 2. Minimum follow-up was 1 year (mean, 15.6 +/- 3.3 months). We recorded the American Shoulder and Elbow Surgeons (ASES) shoulder scores preoperatively and postoperatively. There was no statistical difference in postoperative ASES scores between group 1 (91.5 +/- 10.3) and group 2 (89.2 +/- 15.1) (P =.392). The change in ASES score over time did not differ between the two groups (61.1 vs 60.2, P =.363). In conclusion, within the parameters described above, arthroscopic subacromial decompression does not appear to change the functional outcome after arthroscopic repair of the rotator cuff.