A 69-year-old man with prostate cancer underwent surgery for 16 h. Approximately 6 h after surgery, the patient developed severe pain and motor weakness in his right arm. After neurologic examinations that included a nerve conduction study and electromyography, the patient was diagnosed with a brachial plexus injury. The causes of the brachial plexus injury were thought to be abduction of both arms, direct compression of the shoulder brace, and prolonged surgery. Most of the postoperative peripheral nerve injuries due to patient position are preventable, and anesthetists and surgeons should be very careful in positioning the patient accurately.