The aim of the present study was to evaluate the coexpression of very early (CD69), early (CD25) and late (HLADR) antigens and to analyse the mean fluorescence intensity (MFI) of such activation markers on synovial fluid (SF) and peripheral blood (PB) lymphocytes of patients affected by rheumatoid arthritis (RA) and other types of chronic synovitis (OCS). A three colour cytometric analysis was performed using a peridinin chlorophyll protein conjugated anti-CD3 antibody in combination with fluorescein isothiocyanate or phycoerythrin labelled anti-CD69, anti-HLADR, anti-CD25 monoclonal antibodies (mAbs). A T cell gating method was utilized, so that three sets of bivariant dot plot quadrant displays were obtained (CD69/HLADR, CD69/CD25, CD25/HLADR). A large percentage of SF T lymphocytes in RA showed the coexpression of very early and late activation antigens (CD3 + CD69 + HLADR +), whereas CD3 + CD69 + CD25 + bearing cells and CD3 + CD25 + HLADR + lymphocytes were only a small percentage. Similar results were obtained in patients with OCS, although to a lesser extent. No statistically significant differences in MFI of CD69 and HLADR positive SF T cells between RA and OCS were observed. The CD69 + CD25-HLADR + T cell subset is the most commonly represented in the synovial environment, among those we have evaluated; this phenotype may be characteristic of chronic inflammatory arthritis.