OBJECTIVES: This open-label study is based on a translational approach with the aim of detecting changes in the clinical condition as well as in imaging and synovial biological markers in both synovial fluid (SF) and synovial tissue (ST) in peripheral spondyloarthritis (SpA) patients following intra-articular (IA) blockade of TNF-α by serial etanercept injections. METHODS: Twenty-seven SpA patients with resistant knee synovitis underwent four biweekly IA injections of etanercept (E) (12.5mg). The primary outcome of Thompson's Knee Index (THOMP), and secondary outcomes of Knee Joint Articular Index (KJAI), C-reactive protein (CRP), HAQ-Disability Index (HAQ-DI), maximal synovial thickness (MST) according to ultrasonography (US) and contrast-enhanced magnetic resonance (C+MR) imaging, ST-CD45+ mononuclear cells (MNC) and ST-CD31+ vessels, IL-1β, IL-1Ra and IL-6 levels in SF were assessed at baseline and at the end of the study. RESULTS: At the study end, clinical and imaging outcomes as well as ST and SF biological markers were significantly reduced compared to baseline. There were significant correlations between clinical, imaging and biological markers (CRP with either THOMP, or KJAI, or HAQ-DI or SF-IL-1Ra; US-MST with KJAI, ST-CD45+ with either THOMP, or KJAI, or ST-CD31+, or SF-IL-1β; SF-IL-6 with either THOMP, or KJAI, or SF-IL-1β, or IL-1Ra). CONCLUSIONS: The proof of concept study revealed early improvement either in local and systemic clinical scores, in synovial thickness measures by C+MR and US, or expression of synovial biological markers. CD45+, CD31+ in ST and IL-6 and IL-1β in SF may be considered potential biomarkers of the peripheral SpA response to IA TNF-α blocking.