Prolonged or enhanced expression of the proto-oncogene Lmo2 is associated with a severe form of T-cell Acute Lymphoblastic Leukemia (T-ALL), designated Early T-progenitor ALL (ETP-ALL), that is characterized by the aberrant self-renewal and subsequent oncogenic transformation of immature thymocytes. It has been suggested that Lmo2 exerts these effects by functioning as component of a multi-subunit transcription complex that includes the ubiquitous adapter Ldb1 along with b-HLH and/or GATA family transcription factors; however, direct experimental evidence for this mechanism is lacking. In this study, we investigated the importance of Ldb1 for Lmo2-induced T-ALL by conditional deletion of Ldb1 in thymocytes in an Lmo2 transgenic mouse model of T-ALL. Our results identify a critical requirement for Ldb1 in Lmo2-induced thymocyte self-renewal and thymocyte radiation-resistance and for the transition of pre-leukemic thymocytes to overt T-ALL. Moreover, Ldb1 was also required for acquisition of the aberrant pre-leukemic ETP gene expression signature in immature Lmo2 transgenic thymocytes. Co-binding of Ldb1 and Lmo2 was detected at the promoters of key up-regulated T-ALL 'driver' genes (Hhex, Lyl1, and Nfe2) in pre-leukemic Lmo2 transgenic thymocytes and binding of both Ldb1 and Lmo2 at these sites was reduced following Cre-mediated deletion of Ldb1. Together, these results identify a key role for Ldb1, a non-proto-oncogene, in T-ALL and support a model where Lmo2-induced T-ALL results from failure to down-regulate Ldb1/Lmo2-nucleated transcription complexes which normally function to enforce self-renewal in bone marrow hematopoietic progenitors.