© 2019 by The American Society of Hematology. Zanubrutinib is a potent and highly selective inhibitor of Bruton tyrosine kinase (BTK). In this first-in-human, open-label, multicenter, phase 1 study, patients in part 1 (3 + 3 dose escalation) had relapsed/refractory B-cell malignancies and received zanubrutinib 40, 80, 160, or 320 mg once daily or 160 mg twice daily. Part 2 (expansion) consisted of disease-specific cohorts, including treatment-naive or relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). The primary end points were safety and tolerability, and definition of the maximum tolerated dose (part 1). Additional end points included pharmacokinetics/pharmacodynamics and preliminary efficacy. Reported herein are results from 144 patients enrolled in the dose-finding and CLL/SLL cohorts. No dose-limiting toxicities occurred in dose escalation. Median BTK occupancy in peripheral blood mononuclear cells was >95% at all doses. Sustained complete (>95%) BTK occupancy in lymph node biopsy specimens was more frequent with 160 mg twice daily than 320 mg once daily (89% vs 50%; P = .0342). Consequently, 160 mg twice daily was selected for further investigation. With median follow-up of 13.7 months (range, 0.4-30.5 months), 89 CLL/SLL patients (94.7%) remain on study. Most toxicities were grade 1/2; neutropenia was the only grade 3/4 toxicity observed in >2 patients. One patient experienced a grade 3 subcutaneous hemorrhage. Among 78 efficacy-evaluable CLL/SLL patients, the overall response rate was 96.2% (95% confidence interval, 89.2-99.2). Estimated progression-free survival at 12 months was 100%. Zanubrutinib demonstrated encouraging activity in CLL/SLL patients, with a low incidence of major toxicities. This trial was registered at www.clinicaltrials.gov as #NCT02343120.