['The precise etiology of knee osteoarthritis (KOA) pain remains highly controversial and there is no known effective treatment. Due to the known and suggested effects of neuropeptide Y (NPY) on pain, we have sought to investigate the relationship between the concentration of NPY in synovial fluid of knee, pain of KOA, and structural severity of KOA.', "One hundred KOA patients and twenty healthy participants (control group) were recruited. The pain and the radiographic grade of KOA were assessed separately by Hideo Watanabe's pain score and Tomihisa Koshino's scoring system. Synovial fluid of knee from all participants was collected with arthrocentesis. Radioimmunoassay was used to examine the concentration of NPY in synovial fluid of knee.", "Concentrations of NPY in synovial fluid were significantly higher in KOA patients (124.7\u2009±\u200933.4\u2009pg/mL) compared with controls (64.8\u2009±\u200926.3\u2009pg/mL) (p\u2009=\u20090.0297). According to Hideo Watanabe's pain score, 100 KOA patients were divided into 5 subgroups: no pain (n\u2009=\u200912), mild pain (n\u2009=\u200925), moderate pain (n\u2009=\u200937), strong pain (n\u2009=\u200919) and severe pain (n\u2009=\u20097). Within the KOA group, significantly higher concentrations of NPY were found in each subgroup as pain intensified (no pain 81.4\u2009±\u200911.7\u2009pg/mL, mild pain 99.1\u2009±\u200923.2\u2009pg/mL, moderate pain 119.9\u2009±\u200931.5\u2009pg/mL, strong pain 171.2\u2009±\u200937.3\u2009pg/mL and severe pain 197.3\u2009±\u200941.9\u2009pg/mL). Meanwhile, according to Tomihisa Koshino's scoring system, 100 KOA patients were divided into 3 subgroups: early stage (n\u2009=\u200930), middle stage (n\u2009=\u200953), advanced stage (n\u2009=\u200917). Concentrations of NPY in middle and advanced stage groups of KOA patients were significant higher than early stage group of KOA patients (early stage 96.4\u2009±\u200927.1\u2009pg/mL, middle stage 153.3\u2009±\u200916.9\u2009pg/mL, advanced stage 149.5\u2009±\u200936.7\u2009pg/mL) (p\u2009=\u20090.0163, p\u2009=\u20090.0352). Concentrations of NPY in advanced stage group of KOA patients has no significant difference compare with middle stage group of KOA patients (p\u2009=\u20090. 2175)."] Do levels of neuropeptide Y in synovial fluid relate to pain in patients with knee osteoarthritis?