Recent reports have hinted that it is possible to regenerate CD4⁺CD25⁺ regulatory T cells (Treg) from CD4⁺CD25⁻ cells, a phenomenon termed conversion. We evaluated the relative contribution of this process to the Treg pool by transferring purified populations of CD4⁺ T cells into T cell-deficient mice. We report that conversion of CD25⁻ cells into the CD4⁺CD25⁺Treg pool is minor if other bona fide CD25⁺ Tregs are present. Moreover, in the same hosts, the loss of CD25 expression by a population of Tregs also decreases in the presence of co-injected CD4⁺CD25⁻ cells. Thus, the rate of exchange between CD25⁻ and CD25⁺ T-cell populations is determined by the presence or absence of T-cell competitors. Our results attest for the role of competition in the contribution of different T-cell subsets for the regeneration of the peripheral CD4⁺ T-cell pool during lymphopenia.