Although numerous studies have implicated signal transducers and activators of transcription (STAT)5 in Bcr/Abl-dependent transformation and survival, the role of STAT5 in chronic myelogenous leukemia (CML) has not been fully elucidated. In mice, the CML-like myeloproliferative disorder can occur in the absence of STAT5A/5B, but the repopulating and colony-formation defects of STAT5A/5B double knockout (−/−) bone marrow (BM) cells have made it difficult to separate Bcr/Abl-specific and Bcr/Abl-non-specific effects. To clarify the role of STAT5 signaling in P210BCR/ABL leukemogenesis, we have expressed Bcr/Abl in primary murine STAT5A-deficient (−/−) BM cells, which have no demonstrable intrinsic hematopoietic defects. Interestingly, the absence of STAT5A profoundly decreased the incidence of the pure CML-like disease. Only 24% of wild-type mice reconstituted with P210-transduced STAT5A(−/−) BM cells developed CML, compared to 93% of P210-STAT5A(+/+)- and 83% of P210(+/−)-reconstituted mice. The remainder of P210-STAT5A(−/−)-reconstituted mice developed an acute lymphoblastic leukemia (ALL)-like disease (20%) or a CML/ALL mix (56%), underscoring the relative importance of STAT5 signaling in P210-induced myeloid leukemia compared to P210-induced lymphoid leukemia. There was also a trend for increased latency of the CML-like disease in P210-STAT5A(−/−)-reconstituted mice, with P210-STAT5A(−/−)-reconstituted mice having a median survival of 38 days, compared to only 22 days for P210-STAT5A(+/+) animals. Compared to P210-control cells, primary leukemia cells from P210-STAT5A(−/−) mice had decreased constitutive expression of the STAT5 target genes Bcl-XL and oncostatin M, but preserved expression of pim-1 and cis. Of mice that developed CML, P210-STAT5A(−/−)-reconstituted animals had greater splenomegaly and tended to have higher Bcr/Abl expression than P210-control animals, suggesting that enhanced expression of Bcr/Abl might compensate for the absence of STAT5A in these mice. Importantly, there was no difference in the number of unique P210-STAT5A(−/−) or P210-STAT5A(+/+) leukemic clones by Southern blot, indicating that the reduced incidence of CML in STAT5A (−/−) mice was not due to the decreased availability of CML progenitors. Altogether, these findings indicate that STAT5, particularly STAT5A, plays an important role in the efficient induction of the CML-like disease in mice. A greater understanding of the STAT5 target genes important in CML will provide important insights into the pathophysiology of CML, and may provide additional molecular targets for CML therapy.