To evaluate superoxide anion (O2-), nitrite/nitrate (NO2-/NO3-), and nitrotyrosine (NT) production and the contribution of myeloperoxidase (MPO) to the production of NT-containing proteins in the synovial fluid (SF) of patients with juvenile idiopathic arthritis (JIA). The affected tissues in inflammatory arthritis produce large amounts of nitric oxide (NO) or peroxynitrite (ONOO-) but there are no reports of NO or ONOO- participation in JIA. We also attempted to correlate our findings with variables of disease activity and articular damage. We analyzed 40 patients with JIA, mean age 12.7 years, mean disease duration 7.8 years. O2- production was measured by cytochrome C reduction after incubation of 106 synovial fluid (SF) cells with or without phorbol myristate acetate (PMA), formyl-methionyl-leucyl-phenylalanine (FMLP) or opsonized zymosan. SF and serum NO2-/NO3- levels were measured by Griess reaction; NT was detected by Western blot. Myeloperoxidase (MPO) activity was estimated spectrophotometrically. Clinical and laboratory variables [erythrocyte sedimentation rate, C reactive protein (CRP), and radiological score] and interleukin 6 (IL-6) levels were evaluated. NO2-/NO3- production was greatly enhanced in the joints of JIA patients (54.6 +/- 3.2 micro M) when compared with serum (13.9 +/- 0.6 micro M; p < 0.001). NO2-/NO3- levels in SF were positively correlated with the number of infiltrating lymphomononuclear cells. NT-modified proteins detected in the SF showed a high correlation with radiological score, disease duration, CRP, and IL-6. Our results confirm the increased oxidative stress in children with JIA, suggesting a high in situ production of NO. The positive correlation between the expression of NT-modified proteins and variables of disease activity and damage is additional evidence that nitrogen and oxygen species may be involved in the joint destruction seen in patients with JIA.