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By genetic inactivation of key microRNA biogenesis enzymes, we and others have previously demonstrated the critical requirement of the microRNA pathway for the differentiation and function of Foxp3(+) regulatory T cells. In this study, we identified members of the miR-17 ∼ 92a cluster of microRNAs to be enriched in regulatory T cells. To investigate the function of this microRNA cluster, we deleted the gene specifically in Foxp3(+) cells in mice. We found that miR-17 ∼ 92a is required for the fitness of regulatory T cells, and deficiency impacted at the level of apoptosis and proliferation of these cells. This led to a loss of Foxp3(+) cells over time, particularly in competitive settings, and culminated in a range of immunologic perturbations. Thus, miR-17 ∼ 92a-target interactions are part of the essential microRNA networks that safeguard the regulatory T cell lineage.