Oxidative stress is thought to play a role in the pathogenesis of fibromyalgia. We examined the hypothesis that oxidative stress was increased in patients with fibromyalgia and related to the severity of symptoms. Urinary F(2)-isoprostane excretion was measured in 48 patients with fibromyalgia and compared to those of 96 control subjects. In patients, we examined the association between oxidative stress and symptoms. Patients with fibromyalgia were significantly more symptomatic than control subjects, but urinary F(2)-isoprostane excretion did not differ significantly (2.3+/-1.9 vs. 2.8+/-2.2 ng/mg creatinine, p=0.16). In patients with fibromyalgia, F(2)-isoprostane excretion was associated with fatigue visual analog scale (rho=0.30, p=0.04) but not with pain, quality of life, functional capacity, depression, number of tender points, or overall impact of fibromyalgia. Oxidative stress is not increased in patients with fibromyalgia, but as was previously found in patients with systemic lupus erythematosus, oxidative stress was associated with fatigue.