Septic arthritis of the shoulder is uncommon in adults. We reviewed the cases of eighteen patients who were followed for a minimum of one year. The patients ranged in age from forty-two to eighty-nine years. All but one patient had at least one serious associated disease. Eight patients had had an injection or aspiration of the shoulder before development of the infection. All but one patient had had a delay in diagnosis. At the time of admission to the hospital, the erythrocyte sedimentation rate was always elevated, but the body temperature and white blood-cell count were not. After treatment, the functional result was usually poor: only five patients regained forward flexion to 90 degrees or more, eight patients had no active motion of the glenohumeral joint, and two patients died. Arthrotomy appeared to afford a better result than did repeated aspiration.