Synovial biopsies were obtained by needle arthroscopy in nine patients with active psoriatic arthritis in a clinically involved knee before the initiation of etanercept (Enbrel©; Wyeth Pharmaceuticals, Louvain-la-Neuve, Belgium). All patients were included in a 6-month open-label clinical trial of 25 mg etanercept two times a week . Follow-up biopsies were taken in the same knee after 6 months. Six biopsies were used for histological examination. Frozen sections were stained with H&E or used for immunofluorescense. Sections were fixed with acetone and stained with goat-anti-human phosphorylated ERK, mouse-anti-human phosphorylated p38 or mouse-anti-human phosphorylated JNK (all from Santa Cruz Technologies, Santa Cruz, USA). Cy2-conjugated donkey anti-goat or Cy3-conjugated goat anti-mouse antibodies (Jackson Immunoresearch, West Grove, USA) were used as secondary antibodies. Negative controls were performed with non-specific IgG. Severity of synovitis was assessed with a composite score based on a blinded semi-quantitative score (0–3) of four parameters (lining layer thickness, sublining vascularity, inflammatory cell infiltration and presence of lymphoid aggregates). Activation of MAPKs was quantified with Image J digital analysis software. The area of fluorescent staining due to antibody-linked fluorochrome emission was normalized to cell number based on area of emission by DAPI nuclear staining. For statistical analysis non-parametric Wilcoxon test for paired samples was used.