Background Recent evidence indicates that human regulatory T cells (Tregs) are composed of three distinct subpopulations: CD25(++) CD45RA(+) resting Tregs (rTregs), CD25(+++) CD45RA(-) activated Tregs (aTregs), which are suppressive, and CD25(++) CD45RA(-) cytokine-secreting (Fr III) cells with pro-inflammatory capacity. Objectives To evaluate the dynamic changes in circulating and pulmonary Treg subpopulations in smokers and patients with chronic obstructive pulmonary disease (COPD), and to explore their potential roles in COPD pathogenesis. Methods Blood samples were obtained from 57 never-smokers, 32 smokers with normal lung function and 66 patients with COPD. Bronchoalveolar lavage (BAL) samples were taken from 12 never-smokers, 12 smokers and 18 patients with COPD. The proportions of Treg subpopulations and activated CD8 T cells were evaluated using flow cytometry. Results In peripheral blood, increased proportions of rTregs, aTregs and Fr III cells were found in smokers compared with never-smokers, whereas patients with COPD showed decreased rTregs and aTregs, and significantly increased Fr III cells compared with smokers. The changes in Treg subpopulations, with an overall decrease in the (aTreg+rTreg):(Fr III) ratio, indicated that immune homeostasis favoured inflammation and correlated with enhanced CD8 T-cell activation (r=-0.399, p<0.001) and forced expiratory volume in 1s (FEV1) % predicted value (r=0.435, p<0.001).The BAL (aTreg+rTreg):(Fr III) ratios displayed more robust correlations with FEV1% predicted value (r=0.741, p<0.01) and activation of effector T cells (r=-0.763, p<0.001). Conclusions The imbalance between the anti-inflammatory subsets (aTreg+rTreg) and the pro-inflammatory subset (Fr III) of Tregs may play an important role in COPD progression.