PI3Kδ plays pivotal roles in the maintenance, proliferation and survival of malignant B-lymphocytes. Although not curative, PI3Kδ inhibitors (PI3Kδi) demonstrate impressive clinical efficacy and, alongside other signaling inhibitors, are revolutionizing the treatment of hematological malignancies. However, only limited in vivo data are available regarding their mechanism of action. With the rising number of novel treatments, the challenge is to identify combinations that deliver curative regimes. A deeper understanding of the molecular mechanism is required to guide these selections. Currently, immunomodulation, inhibition of B-cell receptor signaling, chemokine/cytokine signaling and apoptosis represent potential therapeutic mechanisms for PI3Kδi. Here we characterize the molecular mechanisms responsible for PI3Kδi-induced apoptosis in an in vivo model of chronic lymphocytic leukemia (CLL). In vitro, PI3Kδi-induced substantive apoptosis and disrupted microenvironment-derived signaling in murine (Eμ-Tcl1) and human (CLL) leukemia cells. Furthermore, PI3Kδi imparted significant therapeutic responses in Eμ-Tcl1-bearing animals and enhanced anti-CD20 monoclonal antibody therapy. Responses correlated with upregulation of the pro-apoptotic BH3-only protein Bim. Accordingly, Bim-/- Eμ-Tcl1 Tg leukemias demonstrated resistance to PI3Kδi-induced apoptosis were refractory to PI3Kδi in vivo and failed to display combination efficacy with anti-CD20 monoclonal antibody therapy. Therefore, Bim-dependent apoptosis represents a key in vivo therapeutic mechanism for PI3Kδi, both alone and in combination therapy regimes.