OBJECTIVE To define the immunohistologic features of the synovial membrane (SM) of patients with psoriatic arthritis (PA) and to compare them with those of an age- and disease-duration-matched population of patients with rheumatoid arthritis (RA). METHODS Synovial membrane needle biopsy was performed on 15 PA patients with knee involvement (8 had asymmetric oligoarthritis and 7 had symmetric polyarthritis) and on 15 RA controls. Specimens were stained with monoclonal antibodies against T cells (CD3, CD8, CD4, CD45RO), B cells (CD20), macrophages (Mac387, CD14), and cells bearing class II antigens (DAKO-DR). Vascular endothelium was examined using a polyclonal antibody to Factor VIII-related antigen, and adhesion molecule expression was examined using antibodies 1.3B6, 6.5B5, and 1.4C3, which identify endothelial leukocyte adhesion molecule 1 (ELAM-1), intercellular adhesion molecule 1 (ICAM-1), and vascular cell adhesion molecule 1 (VCAM-1), respectively. RESULTS There was significantly less lining layer hyperplasia, fewer macrophages, and a greater number of blood vessels in PA SM than in RA SM: ELAM-1 expression was less intense in PA than in RA SM, while there was no difference in expression of ICAM-1 and VCAM-1. Numbers of B cells, T cells, and T cell subsets (predominantly CD4, CD45RO T cells) were similar in both groups of patients. CONCLUSIONS Our findings demonstrate important differences in the immunohistologic features of PA and RA SM: The PA SM is more vascular, ELAM-1 expression is less intense, and fewer macrophages invade the stroma and migrate to the lining layer than in RA SM: However, the lymphocytic infiltrate in the SM of both groups is similar.