[Objective] To explore disease-associated molecules in rheumatoid arthritis (RA), we analyzed phosphorylation of synovial proteins in RA in comparison with that in osteoarthritis (OA).<BR>[Materials and Methods] Synoviocytes were obtained from three RA patients and three OA patients. Phosphoproteins purified from the synoviocytes were compared by 2D-DIGE between RA and OA. Protein spots with significantly different phosphorylation levels, were identified by mass spectrometry.<BR>[Results] 22 spots showed more than 2-fold phosphorylation and one spot showed less than 1/2 fold phosphorylation in RA compared to OA. We identified 11 phosphoproteins out of the former 22 spots and the latter one spot. We specifically investigated roles of one of the identified proteins, Annexin A4. Interestingly, introduction of a recombinant Annexin A4 protein into OA synoviocytes reduced production of CXCL-1 and IL-8 brought by TNF alpha stimulation.<BR>[Conclusion] We successfully identified multiple synovial phosphoproteins whose amounts were significantly increased in RA compared to OA. Severe synovial inflammation in RA may lead to the increased phosphorylation of Annexin A4, which decreases CXCL-1 and IL-8. Further phosphorylation of Annexin A4 may have therapeutic roles in RA treatment, by reducing chemotaxis of inflammatory cells into synovium.