The recovery of shoulder muscle strength after open acromioplasty was evaluated in 48 patients (27 male, 21 female, mean age 44.3 years) who had undergone open acromioplasty because of stage II impingement syndrome. The isometric strengths of flexion, abduction and external rotation were measured before the operation and at 3, 6 and 12 months postoperatively. The mean preoperative flexion strength of the involved shoulder was 72.6% of that of the uninvolved shoulder, and this increased to 77.1% by 3 months post operation, to 88.3% at 6 and to 88.3% at 12 months. Corresponding abduction strengths were 68.4%, 80.4%, 88.7% and 91.0% and the external rotation strengths were 75.1%, 77.4%, 95.1% and 93.5%, respectively. These recoveries were markedly improved when the cases with poor subjective results at 1 year were not considered. The low preoperative strengths were more pronounced in women than in men, but recovery was better in women. It is concluded that shoulder muscle strengths recover to near normal in 1 year after open acromioplasty.