BACKGROUND The effectiveness of eculizumab (a terminal complement inhibitor) in acetylcholine receptor (AChR) antibody-negative generalized myasthenia gravis (gMG) is unknown. CASE REPORT A female patient was diagnosed with AChR-antibody and muscle-specific kinase (MuSK) antibody-negative gMG in March 2016. In January 2017, the patient was referred for plasma exchange (PLEX) because of continuing symptoms. She was also receiving azathioprine, mycophenolate mofetil, and pyridostigmine (all were continued during subsequent therapies). PLEX (5 sessions over 10 days) was initially effective, but over the following month the patient received PLEX weekly, then twice weekly, followed by 3-times weekly because of worsening symptoms. In April 2018, PLEX was reduced to twice weekly following initiation of eculizumab (weekly induction dose of 900 mg 1 day after first PLEX, plus 600 mg on the day of the second PLEX session, for 4 weeks). The patient was then stabilized on eculizumab 1200 mg every 2 weeks and the frequency of PLEX treatment was reduced, until PLEX was discontinued at Week 39 after eculizumab initiation. During eculizumab treatment, the patient's myasthenia gravis activities of daily living (MG-ADL) score decreased from 9 to 1 or 2 at most assessments, with a transient increase to 4 or 5 between Weeks 19 and 27 following less frequent eculizumab treatment. There were no eculizumab-related adverse events. CONCLUSIONS Following transition from 3-times weekly PLEX to eculizumab in a patient with treatment-refractory, AChR antibody- and MuSK antibody-negative gMG, there were clinically significant improvements in everyday activities affected by MG symptoms. Further investigation of eculizumab in antibody-negative MG is required.