14-3-3 proteins represent a family of ubiquitously expressed intracellular chaperonins consisting of seven different highly conserved isoforms. In 2007, we reported that the eta (η) isoform was present at significantly higher levels in the synovial fluid and serum of patients with arthritis as compared to healthy subjects. That study relied on the semi-quantitative immunoblot approach using a monospecific rabbit polyclonal antibody. Additionally, we reported that there was a significantly strong correlation between the levels of 14-3-3 eta and matrix metalloproteinases (MMPs), and that extracellular 14-3-3 eta possessed ligand-like activity capable of inducing these degradative enzymes underscoring the possible role 14-3-3 eta plays in the pathogenesis of joint damage in RA. The OMERACT soluble biomarker sub-committee has published validation criteria related to truth, discrimination and feasibility for biomarkers reflecting structural damage.[ 2] The large majority of biomarker assays assessed in RA have not undergone such validation, particularly the key assay performance criteria considered essential prior to clinical validation studies. In this study, we present data related to the development and validation of a quantitative 14-3-3 eta ELISA that has been used to investigate the clinical merits of this protein in RA. Rheumatoid arthritis (RA) is a chronic autoimmune disease that if left untreated results in severe joint destruction leading to impaired physical function and work disability. It is now widely accepted that early identification, assessment of the severity of the disease at presentation, together with an early and effective treatment strategy can significantly improve a patient’s prognosis and workplace function.