BACKGROUND: T lymphocytes that bear CD3 but lack CD4 and CD8 (Double negative T cells, DN T cells) are normally present early in ontogeny in the fetal thymus, but constitute only a small proportion of adult thymocytes and peripheral blood. Since DN T TCR alpha beta+ cells have been found to accumulate in the lymphoid organs of lpr and gld mice and to be expanded in patients with autoimmune diseases, their functional properties are now of considerable interest. METHOD: Sixty four rheumatoid arthritis (RA) patients and 24 healthy volunteers were studied from Jan 1997 to Feb 1998. The whole blood from the patients and controls were analyzed by flow cytometry (Elite ESP, Coulter, USA) and XL-II software after the cells were stained with trifluorochrome monoclonal antibodies (anti-CD4-FITC/anti-CD8-PE/anti-CD3-PE-Cy5, anti-CD3-FITC/anti-CD19-PE/anti-CD45-PE-Cy5, anti-CD3-FITC/ CD16+56-PE) (Immunotech, Coulter, USA). We reviewed patient records to find out the inflammatory parameters and Ritchie index. RESULTS: We confirmed the presence of DN T cells in peripheral blood of healthy volunteers and RA patients. DN T cells were lower in RA patients (mean+/-SD; 6.44%+/-4.46), when compared to healthy volunteers (mean+/-SD; 9.97%+/-4.50) (p=0.001). There was no clinical correlations between DN T cells and inflammatory parameters. CONCLUSION: Our study showed that the DN T cells in normal control were about 10% of CD3 positive T cells and the cells were significantly lower in RA patients. Although we did not identify whether these cells have either TCR alpha beta or TCR gamma delta, we could conclude that these cells are not expanded in RA patients. We would like to continue this study further 1) to identify TCR the DN T cells have and 2) to monitor the changes after treatment.