Rheumatoid arthritis (RA) is the most common inflammatory arthritis, characterized by marked infiltration of mononuclear cells including B cells into the inflamed synovium. Anti-glucose-6-phosphate isomerase (GPI) antibody (Ab) is an arthritogenic Ab in K/BxN T cell receptor transgenic mice, and is also present in some patients with RA. To characterize synovial B cells from anti-GPI Ab-positive RA, synovial immunoglobulin (Ig) heavy chain variable regions (VH) were compared with those of negative individuals. Synovial tissues were obtained from six RA patients (three anti-GPI Ab-positive and three anti-GPI Ab-negative). Ig-VH genes were amplified by PCR using family-specific primers and were subsequently sequenced. In synovial B cells from anti-GPI Ab-positive RA patients, VH4 and JH4 were predominantly expressed (p<0.0001). The immunoglobulin heavy chain complementarity-determining region 3 (IgH-CDR3) length in the synovium of anti-GPI Ab-positive individuals was shorter than that in anti-GPI Ab-negative individuals (p=0.0005). In addition, the IgH-CDR3 of anti-GPI Ab-positive patients was rich in basic-ionized amino acids (arginine, histidine, and lysine) near their central position, suggesting a high affinity. Our results support the notion that Ig-VH4 B cells in RA synovium with anti-GPI Ab are affinity-matured and that anti-GPI Ab might be associated with the skewed IgH-CDR3.