Aim: Obinutuzumab induces NK cell antibody-dependent cell-mediated cytotoxicity. Objective: Investigate the effects on the human immune system after obinutuzumab monotherapy treatment in patients with chronic lymphocytic leukemia (CLL). Method: To evaluate these effects, we analyzed the distribution of CD4⁺ and CD8⁺ T cells, B cells and NK cells in the peripheral blood of eight CLL patients who were treated with obinutuzumab in monotherapy. The distribution of peripheral blood lymphocytes was examined prior to each dose of obinutuzumab and 24–72 h after the first 1000 mg complete dose (cycle 1 day 2). We also repeated measurements 3 months after the last obinutuzumab dose. In total we obtained ten samples of each patient. Analyses were performed by flow cytometry with monoclonal antibodies against CD3, CD4, CD8, CD19 and CD56⁺. Results: After the first 1000 mg obinutuzumab infusion (cycle 1 day 2), CD4⁺ T cells and CD8⁺ T cells were significantly decreased in peripheral blood compared with prior to therapy. This reduction in the CD4⁺ T cells persisted after six cycles of obinutuzumab (1235 cells/μl basal vs 662 cells/μl after six cycles, p ≤ 0.05), but not in CD8⁺ T cells (987 cells/μl basal vs 837 cells/μl after six cycles). Interestingly, we also observed significant differences in the NK cell compartment after the first 1000 mg drug infusion (490 cells/μl basal vs 23 cells/μl postinfusion, p ≤ 0.05), and after cycle 6 (490 cells/μl basal vs 149 cells/μl after six cycles, p ≤ 0.05). Conclusion: Obinutuzumab induces depletion of NK cells in CLL.