Background: Myasthenia gravis (MG) is an autoimmune disorder and double-filtration plasmapheresis (DFP) is an effective therapy for MG crisis.Although natural killer (NK) cells are part of the innate immune system, also influence adaptive immune responses.Thus, we examined the effects of DFP on the cytotoxicity of NK cells.Methods: A total of 20 MG patients and 16 healthy controls were recruited for the study.Ficoil-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: DFP treatment decreased significantly the NK cell cytotoxicity in MG patients, especially in good responders, those who were acetylcholine receptor (AChR) antibody positive, and those receiving immunosuppressants.Conclusions: The decrease of NK cell cytotoxicity after DFP was strongly correlated with the decline of AChR antibody titer, which could benefit MG patients.