The modulatory function of individual microRNAs (miRNAs) in Notch-driven T-cell acute lymphoblastic leukemias (T-ALLs) has recently been established. Although protumorigenic and tumor-suppressive miRNAs are implicated in disease onset in murine models of Notch-driven T-cell leukemia, whether Dicer1-processed miRNAs are essential for Notch-driven T-ALL is currently unknown. Here we used conditional and inducible genetic loss-of-function approaches to test whether the development and maintenance of Notch-driven T-ALL was dependent on Dicer1 function. Mice with specific inactivation of both Dicer1 alleles in the T-cell lineage did not develop Notch-driven T-ALL. In contrast, loss of 1 functional Dicer1 allele did not significantly perturb T-ALL onset and tumor progression. Inducible inactivation of Dicer1 in early stage polyclonal T-ALL cells was sufficient to abrogate T-ALL progression in leukemic mice, whereas late-stage monoclonal T-ALL cells were counterselected against loss of Dicer1. Lineage- tracing experiments revealed that Dicer1 deficiency led to the induction of apoptosis in T-ALL cells, whereas cell cycle progression remained unaltered. Through microarray-based miRNA profiling, we identified miR-21 as a previously unrecognized miRNA deregulated in both mouse and human T- ALL. Herein, we demonstrate that miR-21 regulates T-ALL cell survival via repression of the tumor suppressor Pdcd4.