In patients with rheumatoid arthritis, treatment with tofacitinib for up to 6 months was associated with a dose-dependent decrease in circulating CD16/56+ natural killer cells, with the maximum decrease estimated to occur approximately 8–10 weeks after initiation of therapy. These changes generally resolved within 2–6 weeks after discontinuation of therapy. Tofacitinib treatment was associated with a dose-dependent increase in B-cell counts. Changes in circulating T-lymphocytes and T-lymphocyte subsets (CD3+, CD4+, and CD8+) were small and inconsistent.