Sixty patients who were undergoing unilateral knee arthroscopy randomly received unilateral spinal anesthesia with 0.5% hyperbaric bupivacaine 6 mg combined with normal saline 0.05 mL or hydromorphone 2.5, 5 or, 10 μg/0.05 mL. The verbal numerical rating scale (VNRS) pain scores were measured at 30 minutes and 2, 4, 6, 12 and 24 hours postoperatively, and the side effects of hydromorphone were recorded.