Purpose - T-regulatory cells (Tregs) are a sub-population of lymphocytes that act to supress aberrant immune responses. We investigated changes in the numbers of naïve and terminally differentiated Tregs in the peripheral blood to establish their role in the immuno-suppressive response to prolonged exercise. Methods - Blood was drawn from seventeen experienced runners (age, 40 ± 12 yrs; height, 1.75 ± 0.08 m; mass, 71.4 ± 10.8 kg) before, ~1 h after (POST-1h), and on the day following the marathon (POST-1d). Tregs (CD3+CD4+Foxp3+CD25++CD127-) were analysed in peripheral blood mononuclear cells (PBMC) using flow cytometry. The markers CD45RA and HLA-DR were included to define naïve and terminally differentiated Tregs, respectively. Results - The absolute number of Tregs decreased (27%) POST-1h marathon (P0.05). In contrast, an increased number of Tregs expressing HLA-DR was observed at POST-1d (P0.05) but returned to pre-exercise levels POST-1d. The suppressive cytokine, transforming growth factor-beta (TGF-β), was unaffected by the marathon (P>0.05). Conclusions - These results suggest that Tregs do not play a major role in immune suppression in the early hours of recovery from a marathon. However, terminally differentiated HLA-DR+ Tregs are mobilized the following day, which could represent a compensatory attempt by the host to restore immune homeostasis and limit excessive cell damage.