Autoimmune diseases such as rheumatoid arthritis (RA) and Behcet’s disease (BD) feature chronic inflammation, in which excessive levels of reactive oxygen (ROS) and nitrogen (RNS) species are produced by activated inflammatory cells. These may cause tissue damage and contribute to the development or progression of disease, and may also modify cellular proteins. Such oxidative/nitrosative modifications could potentially be utilised as biomarkers of the stage or severity of disease, should such correlations be reliably observed and measurable in patient samples. Previous studies have suggested that increased 3-nitrotyrosine (3-NT), is present in plasma from BD patients [1], and both plasma and synovial fluid in RA patients [2]. 3-NT is formed when peroxynitrite (ONOO-) reacts with accessible tyrosine residues in proteins and is considered to be a marker of excessive inflammation [3]. However, the specificity of the HPLC method used to measure 3-NT in RA has been called into question [4], and the increases in 3-NT in BD were minimal, and their biological significance unproven. We therefore sought to measure 3-NT levels simultaneously in plasma from healthy volunteers and patients with active BD, alongside plasma and matched synovial fluid from patients with RA, by sandwich ELISA. Our aim was to further clarify the levels of 3-NT that can be measured in such samples, and to determine whether the nitration of tyrosine residues acts as a biomarker of inflammatory status. In 3 of 4 tested samples from healthy subjects, 3-NT levels were below the limit of detection of the assay (2.1 nM). The fourth sample was at the lower limit of detection. Thus, as expected, nitrated tyrosine was present at low or undetectable levels in subjects with no apparent sign of inflammatory disease. Surprisingly, 3-NT was not elevated in patients with active BD. None of the samples tested contained detectable levels of 3-NT. In contrast, however, 3-NT levels were markedly elevated in both the plasma and synovial fluid of some RA patients. Two samples showed 3-NT to be undetectable, while another patient had low levels. However, in a further 3 patients, 3-NT was markedly elevated. These samples contained an average of 503.0 ± 226.7 nM 3-NT in synovial fluid, and 784.0 ± 381.0 nM in plasma. In conclusion, elevated 3-NT may be useful as a potential biomarker of some, but not all, inflammatory conditions. Surprisingly, Behcet’s disease is not associated with elevated 3-NT generation, despite being a chronic inflammatory condition.