This study aimed to define the change of serum level of nociceptin in pain patients. Seventy pain patients and 20 normal healthy subjects were enrolled. Patients were divided into three groups according to the duration of pain; (1) acute (within 4 weeks), (2) subacute (4 weeks to 6 months), and (3) chronic (> 6 months) state. Serum concentration of nociceptin was measured by radioimmunoassay. Serum nociceptin level was significantly higher in the patients with pain than in normal healthy subjects ( rho < 0.01). Furthermore, nociceptin level was significantly higher in the chronic pain group than acute group ( rho < 0.05). Serum nociceptin level has a relationship with the existence of pain and its duration.