BCR-ABL and v-ABL are oncogenic forms of the Abl tyrosine kinase that can cause leukemias in mice and humans. ABL oncogenes trigger multiple signaling pathways whose contribution to transformation varies among cell types. Activation of phosphoinositide 3-kinase (PI3K) is essential for ABL-dependent proliferation and survival in some cell types, and global PI3K inhibitors can enhance the antileukemia effects of the Abl kinase inhibitor imatinib. Although a significant fraction of BCR-ABL-induced human leukemias are of B-cell origin, little is known about PI3K signaling mechanisms in B-lineage cells transformed by ABL oncogenes. Here we show that activation of class I_A PI3K and downstream inactivation of FOXO transcription factors are essential for survival of murine pro/pre-B cells transformed by v-ABL or BCR-ABL. In addition, analysis of mice lacking individual PI3K genes indicates that products of the Pik3r1 gene contribute to transformation efficiency by BCR-ABL. These findings establish a role for PI3K signaling in B-lineage transformation by ABL oncogenes. (Blood. 2004;103:4268-4275)