Vγ9/Vδ2 T cells constitute a minor proportion of human peripheral blood T cells that can expand rapidly upon infection with microbial pathogens. Vγ9/Vδ2 T cell numbers change characteristically with age, rising from birth to puberty and gradually decreasing again beyond 30 years of age. In adults, female blood donors have significantly higher levels than males, implying that circulating Vγ9/Vδ2 T cells in women remain elevated for a longer period in life and drop less strikingly than in men. This loss in men is accompanied by a substantial depletion of CD27⁻CD45RA⁻ and CD27⁻CD45RA⁺ effector T cells and a parallel increase in CD27⁺CD45RA⁻ central memory T cells while in women, the distribution of Vγ9/Vδ2 T cell subsets remains virtually unchanged. The phenotypical conversion in men older than 30 years is mirrored by an increased proliferative response of Vγ9/Vδ2 T cells and a reduced interferon-γ secretion upon stimulation with isopentenyl pyrophosphate in vitro.