The purpose of this study was to determine whether imbalance of the internal and external rotator musculature of the shoulder were etiological factors implicated in impingement syndrome. Shoulder torque measurements were obtained from 15 asymptomatic volunteers and 30 patients with chronic impingement syndrome, 15 of whom were evaluated after arthroscopic anterior acromioplasty. Isokinetic strength was assessed using the Biodex system in a modified position (in the plane of the scapula and in 45 degrees abduction) with test speeds of 60 degrees and 180 degrees per second. Internal and external rotator strength values and ratios were calculated for both peak torque and average power. Shoulder rotational strength values and the internal rotator/external rotator ratio were significantly higher in the dominant and nondominant control group shoulders than in the involved and uninvolved impingement shoulders for operated on and nonoperated on patients. These data demonstrate that primary change in the normal internal rotator/external rotator ratio of the shoulder is an etiological factor implicated in impingement syndrome not modified by anterior acromioplasty.