© 2019 The Authors. Muscle & Nerve published by Wiley Periodicals, Inc. Introduction: Eculizumab is effective and well tolerated in patients with antiacetylcholine receptor antibody-positive refractory generalized myasthenia gravis (gMG; REGAIN; NCT01997229). We report an interim analysis of an open-label extension of REGAIN, evaluating eculizumab's long-term safety and efficacy. Methods: Eculizumab (1,200 mg every 2 weeks for 22.7 months [median]) was administered to 117 patients. Results: The safety profile of eculizumab was consistent with REGAIN; no cases of meningococcal infection were reported during the interim analysis period. Myasthenia gravis exacerbation rate was reduced by 75% from the year before REGAIN (P < 0.0001). Improvements with eculizumab in activities of daily living, muscle strength, functional ability, and quality of life in REGAIN were maintained through 3 years; 56% of patients achieved minimal manifestations or pharmacological remission. Patients who had received placebo during REGAIN experienced rapid and sustained improvements during open-label eculizumab (P < 0.0001). Discussion: These findings provide evidence for the long-term safety and sustained efficacy of eculizumab for refractory gMG. Muscle Nerve 2019.