Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by aggressive proliferation of synovial tissue leading to destruction of cartilage and bone. To identify molecules which play a crucial role for the pathogenesis, we compared mRNA expression pattern of RA synovium with that of osteoarthritis (OA), using the differential display. From the panel of differentially expressed genes, ID1 (inhibitor of differentiation 1) was considered to be particularly relevant to the pathogenesis of RA, because Id family genes have been shown to play a role in cell proliferation and angiogenesis. To examine whether the upregulation of these genes is consistently observed in the patients with RA, mRNA levels of ID1 and ID3 in the synovial tissues from 13 patients with RA and 6 patients with OA were semi-quantitatively analyzed by RT-PCR. Mean mRNA levels of ID1 and ID3 were significantly elevated in RA synovia compared with OA by 8.6-fold (P = 0.0044) and 3.3-fold (P = 0.0085), respectively. Immunohistochemistry revealed striking staining of Idl and Id3 in the endothelial cells, suggesting a possible role of Id in severe angiogenesis observed in RA. The expression of Id family genes in the synovium constitutes a new finding of particular interest. Their functional role as well as their contribution to the genetic susceptibility to RA requires further investigation.