OBJECTIVE Decoy receptor 3 (DCR3) was a newly identified soluble receptor which was reported to modulate the function of T cells, dendritic cells and macrophages. The aim of this study was to investigate DCR3 expression on the synovial tissue in different types of arthritis. METHODS We obtained synovial tissues from 17 rheumatoid arthritis (RA), 17 ankylosing spondylitis (AS) and 17 osteoarthritis (OA) patients. Synovial specimens were stained with hematoxylin and eosin. The amount of lymphocytes and mononuclear cells infiltration and vascularity during light microscopic examination was scored from 0-4. The expression of CD3, CD4, CD8, CD68 and DCR3 in lining layer (LL) and sublining layer (SL) cells was stained using the immunohistochemical method and analysed by microscopic examination (score from 0-4, 0=absent, 1=slight, 2=moderate, 3=large, 4=extreme). RESULTS OA patients were older than the RA and AS patients (65.9±10.3 years for OA, 58.4±17.7 for RA, and 43.2±16.4 for AS). Synovial tissues in RA patients had significantly increased mononuclear cells infiltration when compared to AS and OA patients (2.3±0.6, 1.9±0.5, 1.6±0.5, respectively, p<0.05). There was no striking difference in DCR3 expression in the synovial LL between RA, AS, and OA patients. CD4+ T cells and CD68+ monocytes/macrophages in the SL were more prominent in RA and AS than in OA (p<0.05). Similarly, DCR3 in the SL was more overexpressed in RA and AS than in OA (1.83±0.21, 1.71±0.36, 1.39±0.31, respectively, p<0.01). CONCLUSIONS The increased synovial inflammatory cells infiltration in RA and AS was associated with the elevated DCR3 expression.