Septic arthritis of the shoulder, elbow, or wrist is not commonly described in the literature. We evaluated the micro- biology of these infections and the diagnostic role of joint aspirate findings. This retrospective study includes 52 patients, diagnosed from 1994 to 2004, with septic arthritis of the shoulder (n = 17), elbow (n = 23), or wrist (n = 12). The mean age was 44.3 years, and 54% of patients (28/52) had comorbidities. The prevalence of Staphylococcus aureus was 76%. Ninety-six percent of aspirates with positive cultures had a polymorphonuclear differential count greater than 85% of the total aspirate white blood cell count. Staphylococcus aureus is the most prevalent organism. A high index of suspicion for septic arthritis is necessary for all patients with upper extremity joint complaints, irrespective of age and medical status. The joint aspirate differential white blood cell count may be helpful in the diagnosis.