OBJECTIVE Myasthenia gravis (MG) is an autoimmune disorder that may involve natural killer (NK) cells. Although NK cells are part of the innate immune system, they also influence adaptive immune responses. Double-filtration plasmapheresis (DFP) is an effective therapy for MG crisis. Thus, we examined the effects of DFP on the cytotoxicity of NK cells. METHODS A total of 20 patients with MG and 16 healthy controls were recruited for the study. Ficoll-Paque-isolated peripheral blood mononuclear cells (PBMCs) and K562 cells were used as the effector and target cells, respectively. NK cell cytotoxicity was analyzed using flow cytometry immediately before and after DFP and upon course completion. RESULTS Double-filtration plasmapheresis treatment decreased significantly the NK cell cytotoxicity in patients with MG, especially in good responders, those who were positive for acetylcholine receptor (AChR) antibodies, and those receiving immunosuppressants. CONCLUSIONS The decrease in NK cell cytotoxicity after DFP and the decline of AChR antibody titer were observed in good responders indicating that this could benefit patients with MG.