(1) To develop a simple outcome measure of shoulder function in rheumatoid arthritis (RA), the Shoulder Function Assessment (SFA) Scale; (2) to compare the properties of this scale with those of 2 existing measures of shoulder function, the Constant Scale and the Hospital for Special Surgery (HSS) Scale. Fifty consecutive patients with RA participated in an inpatient multidisciplinary treatment program. The SFA Scale was constructed by selecting items considered simple to assess and relevant to shoulder function by a team consisting of a rheumatologist, an orthopedic surgeon, a physical therapist, and an occupational therapist. To examine the intra and interobserver reliability in 25 patients the SFA Scale, the Constant, and the HSS Scale were assessed twice by examiner CHME, in the other 25 patients once by examiner CHME, and once by examiner EMV. The validity of all 3 scales was determined by calculating the correlation with (1) the observed shoulder function, (2) the patient's opinion of shoulder function, and (3) shoulder joint deformity. A receiver operating characteristic curve was constructed to determine the accuracy of all scales to discriminate between differences in the shoulder function of the "best" and "worst" shoulder as reported by the patient. The validity and the reliability of the SFA Scale were equivalent to or better than the validity and reliability of the Constant and the HSS scale. The discriminative ability of the SFA Scale was superior to both other scales. The SFA Scale is a reliable, valid, and accurate measure of shoulder function in patients with RA that can be completed within 3 minutes.