Aims: This study was conducted to investigate the serum level of serotonin (S-5-HT) in patients with temporomandibular disorders (TMD) of muscular origin, ie, localized myalgia, and to compare it to that found in healthy individuals and patients with fibromyalgia. A second aim was to investigate the association between S-5-HT and pain parameters. Methods: Twenty patients with localized myalgia participated in the study. Twenty age- and gender-matched healthy individuals and twenty patients with fibromyalgia served as controls. The participants were examined clinically as to the condition of the temporomandibular region and S-5-HT. Results: The levels of S-5-HT did not differ significantly between the groups. However, in patients with localized myalgi a there was a negative correlation between S-5-HT and tenderness of the temporomandibualr muscles. Conclusion: The results of this study indicate that allodynia of orofacial muscles in patients with TMD is signficantly related to S-5-HT concentration.