In a prospective randomised study we compared the results of arthroscopic subacromial bursectomy alone with debridement of the subacromial bursa followed by acromioplasty. A total of 57 patients with a mean age of 47 years (31 to 60) suffering from primary subacromial impingement without a rupture of the rotator cuff who had failed previous conservative treatment were entered into the trial. The type of acromion was classified according to Bigliani. Patients were assessed at follow-up using the Constant score, the simple shoulder test and visual analogue scores for pain and functional impairment. One patient was lost to follow-up. At a mean follow-up of 2.5 years (1 to 5) both bursectomy and acromioplasty gave good clinical results. No statistically significant differences were found between the two treatments. The type of acromion and severity of symptoms had a greater influence on the clinical outcome than the type of treatment. As a result, we believe that primary subacromial impingement syndrome is largely an intrinsic degenerative condition rather than an extrinsic mechanical disorder.