Transforming growth factor (TGF)-β1, a potent immunoregulatory molecule, was found to control the life and death decisions of T lymphocytes. Both thymic and peripheral T cell apoptosis was increased in mice lacking TGF-β1 (TGF-β1^−/−) compared with wild-type littermates. Engagement of the T cell receptor enhanced this aberrant T cell apoptosis, as did signaling through either the death receptor Fas or the tumor necrosis factor α receptor in peripheral T cells. Strikingly, TGF-β was localized within the mitochondria of normal T cells, and the absence of TGF-β1 resulted in disruption of mitochondrial membrane potential (Δψ_m), which marks the point of no return in a cell condemned to die. This TGF-β–dependent regulation of viability appears dissociable from the TGF-β1 membrane receptor–Smad3 signaling pathway, but associated with a mitochondrial antiapoptotic protein Bcl–XL. Thus, TGF-β1 may protect T cells at multiple sites in the death pathway, particularly by maintaining the essential integrity of mitochondria. These findings may have broad implications not only for T cell selection and death in immune responses and in the generation of tolerance, but also for defining the mechanisms of programmed cell death in general.