Upregulation of vascular cell adhesion molecule-1 (VCAM-1) has been implicated in various pathological conditions. In this study, we used in situ hybridization to determine the cell types expressing VCAM-1 mRNA in rheumatoid synovium. Synovial tissues from seven rheumatoid arthritis (RA) and four osteoarthritis (OA) patients were examined for VCAM-1 mRNA expression. In situ hybridization as well as immunohistochemical analysis showed that VCAM-1 expression was mainly localized to the hyperplastic synovial lining cells and to a lesser extent endothelial cells and synovial fibroblasts in rheumatoid synovium. VCAM-1 expression in OA is less prominent. Hyperplastic synovial lining cells expressing VCAM-1 were mainly CD13+, CD14+, CD33+ and HLA-DR+. Together with morphological features, this suggests that they are type A macrophage-like synovial cells. Our findings indicate that overexpression of VCAM-1 may contribute to the formation of hyperplastic lining layers in the synovium seen in this disorder.