Twenty-five patients with rotator cuff tears had bilateral isokinetic shoulder strength evaluations after a pain-relieving subacromial lidocaine injection. Shoulder strength testing was repeated at six months and again at 12 months after rotator cuff surgery. Strength was recorded as a ratio of peak torques comparing the operative with the nonoperative shoulder. Preoperative strength averaged 37%, 36%, and 33% for abduction, external rotation, and forward flexion. Six-month postoperative strength increased to 68%, 76%, and 66% for abduction, external rotation, and forward flexion, respectively. Twelve-month postoperative strength increased to 104%, 142%, and 97% for abduction, external rotation, and forward flexion. Shoulders with rotator cuff tears demonstrate major objective signs of weakness. Shoulder pain obscures objective evaluation of weakness. Preoperative strength can be accurately measured after subacromial lidocaine injection. Shoulder strength is significantly improved by rotator cuff repair.