To explore such an association, the presence of deiminated fibrin in the ST was assessed in a series of 32 patients, 13 with RA and 19 with non-RA rheumatological disorders (controls). ST biopsies were collected in macroscopically inflamed areas identified under needle arthroscopy of the knee. Histopathological examination confirmed the existence of synovitis in all of the samples. The presence of deiminated fibrin was first assessed by immunoblotting of ST extracts using antibodies to citrullyl residues and to the α and β chains of fibrin, and AhFibA purified from a pool of RA sera. Deiminated fibrin was evidenced in all of the ST samples. Moreover, variations in the ratio of deiminated to total fibrin were not related to diagnosis. Immunohistochemical analysis, performed on adjacent synovial biopsy sections of 19 of the 32 patients, using antibodies to the β chain of fibrin and to citrullyl residues, allowed us to confirm the results obtained by immunoblotting, because deiminated fibrin was detected in six RA patients and six controls.