Background: PI3K-delta is critical for activation, proliferation and survival of B cells and plays a role in homing and retention in lymphoid tissues. PI3Kδ signaling is hyperactive in many B-cell malignancies. Idelalisib is a first-in-class, selective, oral inhibitor of PI3Kδ that reduces proliferation, enhances apoptosis, and inhibits homing and retention of malignant B cells in lymphoid tissues (Lannutti et al, 2011). Ofatumumab (O) is an anti-CD20 monoclonal antibody approved for the treatment of pts with CLL refractory to fludarabine and alemtuzumab. Phase 1 studies demonstrated that idelalisib, as monotherapy or combined with O, is highly active in pts with heavily pretreated CLL: pts experienced profound and rapid regression of lymphadenopathy, reductions in disease-associated chemokines, and durable clinical benefit with an acceptable safety profile (Furman et al, 2012). Methods: This study will enroll 210 pts with CLL previously treated with a purine analog and/or bendamustine, with measurable lymphadenopathy who require treatment for CLL and have disease that is not refractory to ofatumumab, and are expected to benefit from a change in therapy because of CLL progression <24 months since completion of their last prior treatment. Pts are randomized in a 2:1 ratio (Arm A:Arm B). In Arm A, pts receive idelalisib at 150 mg BID continuously in combination with 12 infusions of O at 1000 mg over ~24 weeks (weekly x 8 then monthly x 4). In Arm B, pts receive 12 infusions of O at 2,000 mg over ~24 weeks. Stratification factors address IGHV mutational status, del(17p)/p53 mutation status, and refractory vs relapsed disease. The primary study endpoint is PFS. Secondary endpoints include ORR, lymph node response rate, CR rate, and OS. This is an event-driven trial and primary endpoint evaluation will be based on independent central review. For the primary efficacy analysis, the difference in PFS between the treatment arms will be assessed in the ITT analysis set using Kaplan-Meier methods and the stratified log-rank test. The study opened for enrollment in Dec 2012. Clinical trial information: NCT01659021.