Many of the current approaches to cancer immunotherapy aim to exploit and enhance the natural T cell response against cancer. However, these treatment modalities may fall short of their potential due to the elimination of self-tumor antigen specific T cells from the immune repertoire to guard against autoimmunity. To investigate how the precursory frequency of self-antigen specific T cells shapes therapeutic outcome, our lab developed a model system in which the precursor number of T cells specific for a self/melanoma antigen could be manipulated in mice bearing melanoma tumors through the adoptive transfer of tumor specific T cells. Using this model we had previously determined that bringing the frequency of CD8+ T cells within or slightly above a normal physiologic range favored proliferation and generation of polyfunctional effector T cells and anti-tumor immunity, while dramatically exceeding this threshold resulted in intraclonal competition and an impaired immune response. We adapted this model to investigate CD4+ T cells specific for the tumor-self antigen TRP-1 and demonstrated that clonal abundance dictates the development of CD4+ T cell mediated anti-tumor immunity as well. Tumor specific CD4+ T cells operate within the constraints imposed by intraclonal competition, despite ubiquitous expression of cognate antigen. Unlike CD8+ T cells, the defects in activation and proliferation are uncoupled from the development of effector function. Low physiologic precursor frequencies of self-antigen specific T cells support rapid expansion of the population at the expense of the generation of effector function due to onset of irreversible T cell exhaustion. Despite decreased expansion at high precursor frequencies, tumor specific effector CD4+ T cells accumulate in the periphery and tumor in greater numbers, with a lower proportion of TRP-1 specific Foxp3+ regulatory T cells within the population. Through a mechanism of population-induced positive feedback involving paracrine IFN? sharing and traditional T cell help, we observe intraclonal cooperation resulting in strong Th1 differentiation, increased T cell cytotoxicity, and potent anti-tumor responses. These findings assert that the differential effects of T cell clonal abundance on phenotypic outcome should be considered during the design of adoptive T cell therapies, including the use of engineered T cells.