In a study of 7 patients with rheumatoid arthritis (RA), we characterized the T cell population present in the synovial membrane, thought to be the location where T cells trigger the disease. Synovial membrane lymphocytes from the RA patients were found to have a selective expansion of gamma/delta T cells (8.8% in synovial membrane versus 4% in peripheral blood). Expansion of the gamma/delta T cell subset was not found in the synovial membrane of patients with osteoarthritis. We characterized the gamma/delta T cell repertoire using monoclonal antibodies Ti gamma A, BB3, and delta TCS1, which recognize the gene products encoded by V gamma 9, V delta 2, and V delta 1-J delta 1, respectively. The majority of the synovial gamma/delta T cells did not express the repertoire encoded by these genes, which is found in nearly 100% of the peripheral blood gamma/delta T cells of healthy volunteer donors and in the thymus at early stages of development. We conclude that the synovial membrane of patients with RA displays a selective expansion of a specific population of gamma/delta T cells expressing a clonotypic receptor not yet serologically defined, which might be implicated in the development of the disease.