Metabolism is of central importance to T cell survival and differentiation. It is well known that T cells cannot function in the absence of glucose, but it is less clear how they respond to excessive levels of glucose. In this study we investigated how increasing levels of glucose affect T cell-mediated immune responses. We examined the effects of increased levels of glucose on CD8⁺ T cell behaviour in vitro by assessing activation and cytokine production, as well as oxygen consumption rate, extracellular acidification rate and intracellular signalling. In addition, we assessed in vivo proliferation, cytokine production and cytolytic activity of cells in chemically induced diabetic C57BL6 mice. Elevated levels of glucose in in vitro cultures had modest effects on proliferation and cytokine production, while in vivo hyperglycemia had no effect on CD8⁺ T cell proliferation, interferon gamma production or cytolytic killing.