Septic arthritis relatively uncommonly involves the shoulder, so the diagnosis is often difficult and delayed19. We report a 45-year-old woman with right shoulder pain, initially misdiagnosed as right brachial plexus injury. The correct diagnosis of septic arthritis was only made 6 months later, at which point she had humeral head necrosis requiring hemiarthroplasty. With rehabilitation, she regained a reasonable degree of function. However, early diagnosis and treatment of the infection would have been preferable.