Patient activity level may be an important prognostic variable relating to outcome in patients with shoulder disorders. The purpose of this cross-sectional study was to test the hypothesis that a shoulder activity scale will detect differences in shoulder activity based on the patient diagnosis and age. A total of 157 patients of four academic orthopedic surgeons with a diagnosis of rotator cuff disease (86), glenohumeral instability (31) or osteoarthritis (40) were asked to fill out a previously validated shoulder activity scale and the ASES shoulder questionnaire. The patient’s diagnosis, gender and age were recorded. The activity level was significantly higher in patients with instability than in patients with rotator cuff disease or osteoarthritis. Patients with rotator cuff disease were significantly more active than patients with osteoarthritis. Linear regression of activity level on patient age and diagnosis demonstrated a statistically significant relation- ship between activity level and both independent variables. Diagnosis influenced activity level independent of age. This is the first study to document diagnosis-based differences in the shoulder activity level of patients with different shoulder disorders. Shoulder activity level should be evaluated as a prognostic variable in studies of patients with shoulder disorders that may predict outcome and guide treatment in these patients.