Objective: The aim of the study was to investigate serum levels of asymmetric dimethylarginine (ADMA), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and plasma levels of 8-iso-prostaglandin F2α (8-iso-PGF2α) in patients with fibromyalgia. Design and method: Twenty-seven patients with fibromyalgia and twenty healthy controls were enrolled in this study. ADMA, TNF-α, IL-6 and 8-iso-PGF2α levels were measured by enzyme-linked immunosorbent assay (ELISA). Results: Serum levels of ADMA and TNF-α and plasma levels 8-iso-PGF2α were significantly increased in patients with fibromyalgia compared to controls. However, no significant difference was observed in IL-6 levels between the two groups. ADMA concentrations were positively correlated with TNF-α and 8-iso-PGF2α levels in patients with fibromyalgia. Conclusion: This is the first study reporting that ADMA levels are significantly elevated in patients with fibromyalgia in association with increased 8-iso-PGF2α and TNF-α concentrations. Thereby, ADMA could be suggested as a reliable marker of endothelial dysfunction in patients with fibromyalgia. © 2011 The Canadian Society of Clinical Chemists.