Local glucocorticosteroid injections are frequently used in the treatment of subacromial impingement syndrome (SIS), however its efficacy is still controversial. To compare the efficacy of the subacromial injection of methylprenisolone acetate plus lidocaine (MPL) versus lidocaine alone (LA) in SIS. Consecutive SIS subjects, defined as a positive Neer's injection test were randomized to a subacromial injection of 2 mL methylprednisolone acetate (40 mg/mL) plus 1 mL of 1% lidocaine (27 patients); or 3 mL of 1% lidocaine (29 patients) were studied. The change from baseline of the score of a Spanish validated version of the Shoulder Disability Questionnaire (S-SDQ), pain intensity, and shoulder range of motion were measured at 15 and 30 days, and afterward every month for five months. After adjusting for duration of symptoms and pain intensity at baseline by way of a general lineal model,we did not find differences in the change of S-SDQ scores and shoulder range of motion between the study groups. Subjects randomized to LA had greater improvement of pain intensity than MPL subjects during the entire follow-up. A subacromial injection of methylprednisolone acetate was not more efficacious than the injection of lydocaine alone in patients with SIS. Copyright © 2008 Elsevier España. Reumatología Clínica ® Sociedad Española de Reumatología and ® Colegio Mexicano de Reumatología. Published by Elsevier Espana. All rights reserved.