There are many measurement tools for assessing patients' shoulder symptoms (pain) and function (what patients can do), but they do not measure activity (how often a patient engages in activity). This is relevant because activity level can have an important impact on a patient's outcome. Our goal was to develop a short, easy to administer measure of shoulder activity which could be used to predict outcome of shoulder disorders. The activity scale was developed using established principles: item generation, item reduction, pretesting, and reliability and validity testing. The activity rating is a numerical sum of scores for five activities rated on a five-point frequency scale from never performed (0 points) to daily (4 points). Patients were scored on the following criteria: carrying an object 8 lb or heavier by hand, handling objects overhead, weight training with arms, swinging motion (ie, hitting tennis or golf ball), and lifting objects 25 lb or heavier. Two additional multiple choice questions provide a score assessing participation in contact and overhead sports. The activity scale showed excellent reliability and construct validity. It can be completed quickly and used in conjunction with patient-based measures of shoulder outcome to define patient populations for cohort studies, and to assess activity level as a prognostic factor in patients with shoulder disorders.