Forty-two patients with subacromial impingement syndrome entered a randomized prospective study comparing open anterior acromioplasty with a physiotherapy regime. The criterion for a successful outcome of treatment was a reduction of the initial pain score of more than 50% using the visual analogue scale (VAS) technique. The evaluation was performed by an independent observer. At the 6-month follow-up, treatment in the surgical group had been successful in 12/21 (57%) patients versus 6/18 (33%) in the physiotherapy group. A one-year evaluation revealed 16/21 (76%) "successes" in the surgical group. A direct and unbiased comparison with the physiotherapy group was not possible at one year, since 13 patients chose surgery after initial physiotherapy. However, in "an intention to treat" analysis outcome at one year was significantly better in those randomized to surgery. We found two standardized, composite, active movements--the "Pour out of a Pot" manoeuvre requiring the emptying of a pot of water, and the "Hand in Neck" manoeuvre--to be of predictive value for the outcome of surgery. When combining three pain-related variables into a criterion for prediction of success, a sensitivity of 78% (p < 0.02) and a specificity of 90% (p < 0.03) were attained. We advance the hypothesis that pain in the impingement syndrome is mainly elicited by comparison of the subacromial bursa. In some patients a traction-responsive pain generator in the supraspinatus tendon may be present as well.