Abstract: Increased secretion of tumor necrosis factor-alpha (TNF-α), along with interleukin-1 (IL-1) and interleukin-6 (IL-6), is important in the pathogenesis of rheumatoid arthritis (RA). T regulatory CD4+CD25+ cells play a role in maintaining self-tolerance by downregulating Th1-induced proinflammation. This function has been found to be altered in active RA, whereas anti-TNF-α therapy has been found to improve the suppressive abilities of these cells. Our objectives were to investigate whether T regulatory cells in patients with active RA display a higher sensitivity to spontaneous apoptosis than in normals, and to look into the potential of infliximab (anti-TNF-α therapy) to reduce the sensitivity of these cells to spontaneous apoptosis. Seventeen patients suffering from active RA, having failed multiple disease-modifying antirheumatic drug (DMARD) therapies, were treated with infliximab. Spontaneous apoptosis (as detected by annexin V binding) was determined in all patients and compared with a group of normal individuals at baseline and after three months on infliximab treatment. Peripheral blood mononuclear cells were incubated in 24-well plates at 1 × 106 cells/mL for 48 hours. Annexin V binding on CD4+CD25+ was assessed using three-color assay by flow cytometry. Prior to infliximab initiation, spontaneous apoptosis of T regulatory cells from active RA patients was found to be increased in comparison with controls (26 ± 4.2% vs. 19.8 ± 4.8%, respectively; P= 0.01). Three months later (while still on infliximab) spontaneous apoptosis was comparable in the two groups (20.7 ± 5.2% vs. 20.9 ± 3.4%; P 5 0.8). The absolute number of CD4+CD25+ cells/mL in the peripheral blood at baseline was reduced in 11 out of 17 active RA patients when compared with that of the control group (24 ± 7 vs. 32 ± 11, respectively; P= 0.02). Following anti-TNF-α therapy, CD4+CD25+ cell counts of patients were equivalent to those of normals. The alteration and reversal in both spontaneous apoptosis and cell count of T regulatory cells was found to correlate with RA disease activity. CD4+CD25+ T regulatory cells display increased proclivity to undergo spontaneous apoptosis in active RA. Alterations in CD4+CD25+ cell apoptosis and cell count were found to correlate with RA disease activity. Reversal of these deviations from normal was documented in association with the beneficial outcome of infliximab therapy.