The oncogenic fusion protein E2A–HLF is a chimeric transcription factor that arises from the t (17; 19) translocation in childhood B-cell acute lymphoblastic leukemias (B-precursor ALL) and is associated with very poor outcome. We show that retroviral-mediated expression of E2A–HLF alone is sufficient to immortalize primary lymphoid progenitors. We identify Lmo2 and Bcl-2 as direct target genes downstream of E2A–HLF. We use real-time PCR analysis to show that LMO2 and BCL-2 expression is preferentially upregulated both in biopsy material from t (17; 19) B-precursor ALL patients and lymphoid cell lines derived from t (17; 19) leukemias. Co-expression of Lmo2 and Bcl-2 was sufficient to immortalize lymphoid progenitor cells resulting in a similar phenotype to that induced by E2A–HLF alone. Both shRNA-mediated knockdown of Lmo2 expression and pharmacological inhibition of BCL-2 function in E2A–HLF immortalized cells severely compromised their viability. These data suggest that both Lmo2 and Bcl-2 are required for the action of E2A–HLF in leukemogenesis.