Increased absolute lymphocyte count (ALC) is a key feature of chronic lymphocytic leukemia (CLL) but is also observed during treatment with B-cell receptor pathway inhibitors including ibrutinib, a first-in-class inhibitor of Bruton's tyrosine kinase. In patients with CLL treated with single-agent ibrutinib in two multicenter, open-label, randomized, phase 3 studies (RESONATE-2, NCT01722487; RESONATE, NCT01578707), lymphocytosis was observed in 77 of 136 (57%) patients treated in first-line and 133 of 195 (69%) relapsed/refractory patients. On treatment, lymphocytosis resolved in 95% of patients in the first-line and 94% in the relapsed/refractory setting. The median duration of lymphocytosis was 12 and 14 weeks in the first-line and relapsed/refractory settings, respectively. Lymphocytosis is a common and predictable pharmacodynamic effect of ibrutinib treatment, and in the absence of other signs of progression, does not represent disease progression. Lymphocytosis resolves in the majority of patients and does not require interruption or discontinuation of ibrutinib therapy.