Ageing is associated with evidence of immune deficiency and dysregulation. Key changes in the immune system with ageing include a progressive reduction in naive T cell output associated with thymic involution and peripheral expansion of oligoclonal memory T cells. These features are associated with evidence of impaired immune responsiveness both in vitro and in vivo, termed immune senescence. CD4⁺ CD25⁺ T cells have recently been recognized as mediators of peripheral immune regulation and play a role in the control of autoimmune and pathogen-specific immune responses. The significance of CD4⁺ CD25⁺ regulatory T cells in the context of immunosenescence is not known. We have investigated the number, phenotype and function of CD4⁺ CD25⁺ T cells in healthy volunteers over a wide age range. We demonstrate that the number of CD4⁺ CD25⁺ and CD4⁺ CD25^high T cells in healthy volunteers increases with age. In both age groups CD4⁺ CD25⁺ T cells showed a phenotype consistent with that described for regulatory T cells. Further analysis of CD4⁺ CD25^high T cells in young and elderly donors showed equivalent expression of intracellular CTLA-4 and surface expression of activation markers. In vitro, functional titration assays of CD4⁺ CD25^high T cells demonstrated equivalent regulatory function in both young and elderly donors, with suppression of proliferation and cytokine production in response to polyclonal T cell stimulation. These observations demonstrate an increase in peripheral blood CD4⁺ CD25^high regulatory T cells associated with ageing. The relevance of these expanded cells in relation to the immune senescence seen in the elderly as yet remains unclear.