Leukocyte adhesion to endothelium plays an important role in the development and perpetuation of chronic inflammatory diseases such as rheumatoid arthritis (RA). In order to help define the role of adhesion molecules in arthritic disorders, we have studied the expression of CD11c, endothelial leukocyte adhesion molecule-1 (ELAM-1), vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 in synovial tissues from patients with RA and osteoarthritis (OA) by immunohistochemistry. CD11c is expressed predominantly on macrophages deep within RA and OA synovial tissues, as well as on some synovial tissue lining cells. ELAM-1 has endothelial reactivity, being present mainly on venules and capillaries and staining more blood vessels in RA than OA. VCAM-1 is present predominantly on synovial tissue macrophages and, to a lesser degree, on synovial tissue endothelial cells of venules, capillaries, and arterioles in both RA and OA. Like ELAM-1, VCAM-1 appears to be present more often on endothelial cells in RA than in OA tissues. VCAM-1 is present on macrophages isolated from RA synovium as well as macrophages in situ. Intercellular adhesion molecule-1 is more broadly distributed than the other adhesion molecules, being found on endothelium, macrophages, some fibroblasts, and some lymphocytes in both RA and OA tissues. This study shows that ELAM-1, a molecule that was previously thought to be important mainly in acute inflammatory reactions, is also found in RA, a chronic inflammatory disease, as well as in OA. Thus, ELAM-1 as well as VCAM-1 and intercellular adhesion molecule-1 may be involved in mediating the leukocyte traffic into RA and OA synovium.