Previously, we demonstrated the presence of at least two distinct subpopulations of patients with rheumatoid arthritis (RA) employing a cell-transfer experiment using severe combined immunodeficient (SCID) mice. One group of patients, whose T cells derived from the rheumatoid joints, induced synovial hyperplasia (SH) in the SCID mice (the positive group). The other group did not display the induction of SH (the negative group). TCR/Vβ gene usage analysis indicated that some dominant T cell subpopulations were oligoclonally expanding only in the rheumatoid joints, and not in the periphery of the patients of the positive group. Moreover, these T cell subpopulations were not seen in the joints of patients in the negative group or in non-RA patients. In addition, the preferential uses of certain TCR/Vβs (Vβ8, Vβ12, Vβ13, and Vβ14) genes were demonstrated in these T cells. In this study, to investigate whether these T cells are driven by a certain antigen(s), the third complementarity determining regions (CDR3s) of TCR/Vβ, especially Vβ8 and Vβ14 PCR products, were cloned and sequenced. As a result, a dominant CDR3 sequence, CASS-PRERAT-YEQ, was found in Vβ14+ T cells from the rheumatoid joint of a patient (Patient 1) of the positive group with a Vβ14 skew. The identical CDR3 sequence also predominated in Vβ14+ T cells from the rheumatoid joint of another patient (Patient 7) of the positive group with a Vβ14 skew. In addition, in the patients (Patients 4, 7, 8) of the positive group with a Vβ8 skew, other dominant CDR3 sequences, CASS-ENS-YEQ and CASS-LTEP-DTQ, were found as in the case of Vβ14. However, no identical CDR3 sequences were detected dominantly in the joints of the patients in the negative group or in non-RA patients. A Vβ14+ T cell clone (TCL), named G3, with the identical CDR3 sequence, CASS-PRERAT-YEQ, was isolated successfully from Patient 1, and cell transfer of G3 with autologous irradiated peripheral mononuclear cells induced SH in the SCID mice. Taken together, these results suggest that T cells inducing SH, thought to be pathogenic for RA, might be driven by a certain shared antigen(s).