Natural regulatory T cells (T_reg) represent a distinct lineage of T lymphocytes committed to suppressive functions, and expression of the transcription factor Foxp3 is thought to identify this lineage specifically. Here we report that, whereas the majority of natural CD4⁺Foxp3⁺ T cells maintain stable Foxp3 expression after adoptive transfer to lymphopenic or lymphoreplete recipients, a minor fraction enriched within the CD25⁻ subset actually lose it. Some of those Foxp3⁻ T cells adopt effector helper T cell (T_h) functions, whereas some retain “memory” of previous Foxp3 expression, reacquiring Foxp3 upon activation. This minority “unstable” population exhibits flexible responses to cytokine signals, relying on transforming growth factor-β to maintain Foxp3 expression and responding to other cytokines by differentiating into effector T_h in vitro. In contrast, CD4⁺Foxp3⁺CD25^high T cells are resistant to such conversion to effector T_h even after many rounds of cell division. These results demonstrate that natural Foxp3⁺ T cells are a heterogeneous population consisting of a committed T_reg lineage and an uncommitted subpopulation with developmental plasticity.