P>The relative roles that ageing and lifelong cytomegalovirus (CMV) infection have in shaping naive and memory CD4+ T-cell repertoires in healthy older people is unclear. Using multiple linear regression analysis we found that age itself is a stronger predictor than CMV seropositivity for the decrease in CD45RA+ CD27+ CD4+ T cells over time. In contrast, the increase in CD45RA- CD27- and CD45RA+ CD27- CD4+ T cells is almost exclusively the result of CMV seropositivity, with age alone having no significant effect. Furthermore, the majority of the CD45RA- CD27- and CD45RA+ CD27- CD4+ T cells in CMV-seropositive donors are specific for this virus. CD45RA+ CD27- CD4+ T cells have significantly reduced CD28, interleukin-7 receptor alpha (IL-7R alpha) and Bcl-2 expression, Akt (ser473) phosphorylation and reduced ability to survive after T-cell receptor activation compared with the other T-cell subsets in the same donors. Despite this, the CD45RA+ CD27- subset is as multifunctional as the CD45RA- CD27+ and CD45RA- CD27- CD4+ T-cell subsets, indicating that they are not an exhausted population. In addition, CD45RA+ CD27- CD4+ T cells have cytotoxic potential as they express high levels of granzyme B and perforin. CD4+ memory T cells re-expressing CD45RA can be generated from the CD45RA- CD27+ population by the addition of IL-7 and during this process these cells down-regulated expression of IL-7R and Bcl-2 and so resemble their counterparts in vivo. Finally we showed that the proportion of CD45RA+ CD27- CD4+ T cells of multiple specificities was significantly higher in the bone marrow than the blood of the same individuals, suggesting that this may be a site where these cells are generated.