Monoclonal antibodies and flow cytometry techniques were used to analyze and compare the distribution of lymphocyte subpopulations of peripheral blood and synovial fluid (SF) from 70 patients, 43 with rheumatoid arthritis (RA), 10 with ankylosing spondylitis (AS) or reactive synovitis, 10 with psoriatic arthritis and 7 with other inflammatory arthritic diseases. Patients with RA had significantly reduced number of CD8+ T cells and greater CD4/CD8 ratios in peripheral blood, a greater number of CD4+ T cells and lower CD4/CD8 ratio in SF. No significant difference was found between the groups with AS, reactive synovitis and psoriatic arthritis. The simultaneous analysis of peripheral blood and SF lymphocyte subpopulations allowed us to establish a transsynovial lymphocytic ratio which reflects CD4/CD8 variations on both peripheral blood and SF, 2 easily accessible compartments for physicians. This new ratio may distinguish RA from other inflammatory arthritic diseases.