Reaction of nitric oxide (NO^•) with superoxide radical generates peroxynitrite, which can decompose to products that nitrate aromatic amino acids. Such nitro‐aromatics may be ‘markers’ of NO^•‐dependent oxidative damage. Blood serum and synovial fluid from patients with the inflammatory joint disease rheumatoid arthritis contain 3‐nitrotyrosine. By contrast, body fluids from normal subjects and patients with osteoarthritis contain no detectable 3‐nitrotyrosine; much lower levels were found in serum from patients in the early stages of rheumatoid arthritis. This is evidence that NO^• plays a role in joint damage in rheumatoid arthritis.