The impingement test, placement of local anesthetic in the subacromial space, is considered a useful tool in diagnosing impingement syndrome. The purpose of this study was to examine the predictive value of the impingement test with respect to outcome after arthroscopic subacromial decompression. Fifty-five patients who had a preoperative impingement test were evaluated at 3 and 12 months postoperatively. We noted 88% satisfactory results in patients in whom the impingement test was positive, with only 63% satisfactory results at 3 months and 60% satisfactory results at 12 months in patients in whom the impingement test was negative. Although workers' compensation patients tended to have lower scores than others, the impingement test result was more predictive of outcome than was compensation status. Impingement test results and preoperative American Shoulder and Elbow Surgeons scores were independent predictors of postoperative American Shoulder and Elbow Surgeons scores. Our evidence indicates that the impingement test can be used as a predictor of outcome for patients with impingement syndrome treated by arthroscopic subacromial decompression.