The aim of this study was to investigate the relation between serotonin in the synovial fluid (SF-5-HT) and pain of arthritic temporomandibular joints (TMJ). The study comprised 1 male and 10 female patients (22 TM joints) with bilateral TMJ pain or tenderness and a mean age of 42 years. The patients were studied regarding pain from the TMJ at rest and during mandibular movements as well as regarding pressure pain thresholds and tolerance levels over the TMJ. TMJ samples, obtained by saline washing of the joint, and blood serum samples were taken for measurement of the 5-HT content. There was a positive correlation between SF-5-HT and pain in the TMJ upon movement. Maximum voluntary mouth opening ability was negatively correlated to SF-5-HT. In conclusion, the results of this study indicate that 5-HT in the TMJ synovial fluid is related to pain perceived upon movement of the joint and to decreased mandibular mobility.