Background: Chronic graft versus host disease (cGVHD) is a common complication of allogeneic stem cell transplantation, with pathophysiology involving alloreactive and dysregulated T and B cells and innate immune populations. Ibrutinib, a first-in-class, once-daily inhibitor of Bruton's tyrosine kinase, is indicated by the US FDA for the treatment of patients (pts) with CLL/SLL. Ibrutinib recently received breakthrough therapy and orphan drug designation for the treatment of pts with cGVHD who did not respond to one or more lines of systemic therapy. Ibrutinib reduces severity of cGVHD in murine models and recently was shown to achieve an NIH-defined overall response rate of 67% in pts with steroid relapsed/refractory cGVHD (Miklos Blood 2016). Methods: The primary objective of this Phase 3, multicenter, international, randomized, controlled, double-blind study is to evaluate the 24-week response rate of ibrutinib versus placebo in combination with prednisone. Pts with newly diagnosed moderate or severe cGVHD, as per NIH Consensus Development Project Criteria (2014), will be randomized in a 1:1 ratio to receive either oral ibrutinib (arm A) or placebo (arm B) in combination with oral prednisone. Ibrutinib or placebo will be given until unacceptable toxicity, relapse of underlying disease, death, or the need for a new systemic treatment for progressive cGVHD. Eligible study pts (age ≥12 yrs) must require systemic treatment with corticosteroids and have no prior systemic treatment for cGVHD. The primary endpoint is response rate (complete or partial response) at 24 weeks, as per NIH Consensus Development Project Criteria, and must occur in the absence of both new therapy for cGVHD and relapse/return of the underlying disease that was the indication for transplant. Secondary endpoints will assess for additional clinical benefit including corticosteroid dose reduction, improvement of Lee cGVHD Symptom Scale scores, withdrawal of all immunosuppressants, and overall survival. This study is currently enrolling pts. Funding source: Pharmacyclics LLC, an AbbVie Company. Clinical trial information: NCT02959944