This study was designed to determine the efficacy of filtration leukocytapheresis (LCAP) in the treatment of rheumatoid arthritis (RA) and the mechanism of its efficacy. Three filtration LCAP procedures in 22 RA patients were performed. Heparinized samples were collected from peripheral blood and synovial fluid. The surface markers of T cells were measured by flow cytometry before the first and third procedures. Proportions of activated and memory T cells in the peripheral blood were paradoxically higher after the third procedure than they were prior to the first treatment. Inversely, the activated T-cell counts in the synovial fluid decreased after the third procedure. In RA patients, higher proportions of activated T lymphocytes are present in RA affected joints than in the peripheral blood. If LCAP induced the redistribution of activated T cells from the affected joints into the circulating blood, the effectiveness of LCAP in the treatment of RA might be explained by the mechanism.