A prospective, randomized, controlled, double-blind clinical study was performed to determine the short-term efficacy of subacromial injection of corticosteroids for the treatment of subacromial impingement syndrome. Forty patients were randomized to receive either six milliliters of 1 per cent lidocaine without epinephrine (the control group) or two milliliters containing forty milligrams of triamcinolone acetonide per milliliter with four milliliters of 1 per cent lidocaine without epinephrine (the corticosteroid group). The patients were re-examined serially until completion of the study. Nineteen patients, whose mean age was fifty-six years (range, thirty-two to eighty years), were randomized to the corticosteroid group, and twenty-one patients, whose mean age was fifty-seven years (range, thirty-two to eighty-one years), were randomized to the control group. The mean duration of symptoms before the injection was eight months for both groups. Eighteen patients in the corticosteroid group and nineteen patients in the control group had moderate or severe pain before the injection. At the most recent follow-up evaluation, at a mean of thirty-three weeks for the corticosteroid group and twenty-eight weeks for the control group, three patients in the corticosteroid group had moderate or severe pain, compared with fifteen patients in the control group. The mean active range of forward elevation and external rotation improved by 24 and 11 degrees, respectively, for the corticosteroid group and by 10 and 5 degrees, respectively, for the control group. We concluded that subacromial injection of corticosteroids is an effective short-term therapy for the treatment of symptomatic subacromial impingement syndrome. The use of such injections can substantially decrease pain and increase the range of motion of the shoulder.