Glucocorticoids (GCs) induce apoptosis in lymphocytes and are effective agents for the treatment of leukemia. The activated glucocorticoid receptor initiates a transcriptional program leading to caspase activation and cell death, but the critical signaling intermediates in GC-induced apoptosis remain largely undefined. We have observed that GC induction of the three major protein products of the Bcl-2 relative Bim (BimEL, BimS, and BimL) correlates with GC sensitivity in a panel of human precursor B-cell (pre-B) acute lymphoblastic leukemia (ALL) cell lines. To test the hypothesis that Bim facilitates GC-induced apoptosis, we reduced BIM mRNA levels and Bim protein levels by RNA interference in highly GC-sensitive pre-B ALL cells. Reducing Bim proteins by either electroporation of synthetic small interfering RNA (siRNA) duplexes or lentivirus-mediated stable expression of short hairpin RNA inhibited the activation of caspase-3 and increased cell viability following GC exposure. We also observed that the extent of GC resistance correlated with siRNA silencing potency. siRNA duplexes that reduced only BimEL or BimEL and BimL (but not BimS) exhibited less GC resistance than a potent siRNA that silenced all three major isoforms, implying that induction of all three Bim proteins contributes to cell death. Finally, the modulation of GC-induced apoptosis caused by Bim silencing was independent of Bcl-2 expression levels, negating the hypothesis that the ratio of Bim to Bcl-2 regulates apoptosis. These results offer evidence that the induction of Bim by GC is a required event for the complete apoptotic response in pre-B ALL cells.