BACKGROUND: Active suppression by CD4+CD25+ regulatory T lymphocytes plays an important role in the down-regulation of T cell responses to foreign and self-antigens. This study was conducted to analyze whether the CD4+CD25+ regulatory T cells exist and function normally in tuberculous pleural effusion. METHODS: The percentages of CD4+CD25+ T cells in pleural effusion and peripheral blood from patients with tuberculous pleurisy and peripheral blood from healthy control subjects were determined by flow cytometry. The expression of forkhead transcription factor Foxp3 was also examined. CD4+CD25+ and CD4+CD25(-) T cells from pleural effusion and blood were isolated, and were cultured to observe the effects of CD4+CD25+ T cells on proliferation response of CD4+CD25(-) T cells in vitro. RESULTS: There were increased numbers of CD4+CD25+ T cells in tuberculous pleural effusion compared with peripheral blood from both patients with tuberculous pleurisy and normal subjects, and these cells demonstrated a constitutive high-level expression of Foxp3. Moreover, CD4+CD25+ T cells mediated potent inhibition of proliferation response of CD4+CD25(-) T cells. CONCLUSION: The increased CD4+CD25+ T cells in tuberculous pleural effusion express a high level of Foxp3 transcription factor, while potently suppressing the proliferation of CD4+CD25(-) T cells.