The regulation of human Th17 cell effector function by Treg cells (regulatory T-cells) is poorly understood. In the present study, we report that human Treg (CD4⁺CD25⁺) cells inhibit the proliferative response of Th17 cells but not their capacity to secrete IL (interleukin)-17. However, they could inhibit proliferation and cytokine production by Th1 and Th2 cells as determined by IFN-γ (interferon-γ) and IL-5 biosynthesis. Currently, as there is interest in the role of IL-17-producing cells and Treg cells in chronic inflammatory diseases in humans, we investigated the presence of CD4⁺CD25⁺ T-cells and IL-17 in inflammation in the human lung. Transcripts for IL-17 were expressed in mononuclear cells and purified T-cells from lung tissue of patients with chronic pulmonary inflammation and, when activated, these cells secrete soluble protein. The T-cell-specific transcription factors RORCv2 (retinoic acid-related orphan receptor Cv2; for Th17) and FOXP3 (forkhead box P3; for Treg cells) were enriched in the T-cell fraction of lung mononuclear cells. Retrospective stratification of the patient cohort into those with COPD (chronic obstructive pulmonary disease) and non-COPD lung disease revealed no difference in the expression of IL-17 and IL-23 receptor between the groups. We observed that CD4⁺CD25⁺ T-cells were present in comparable numbers in COPD and non-COPD lung tissue and with no correlation between the presence of CD4⁺CD25⁺ T-cells and IL-17-producing cells. These results suggest that IL-17-expressing cells are present in chronically inflamed lung tissue, but there is no evidence to support this is due to the recruitment or expansion of Treg cells.