Background: In the randomized, double-blind, placebo-controlled, phase 3 PREVENT trial (NCT01892345), eculizumab was well tolerated and significantly reduced relapse risk vs placebo in patients with aquaporin-4 immunoglobulin G-positive (AQP4+) neuromyelitis optica spectrum disorder (NMOSD). The treatment effect observed in a prespecified subgroup of patients who received eculizumab monotherapy vs placebo alone (i.e. without concomitant immunosuppressive therapy [IST]) was consistent with the overall population. Objectives: To examine the long-term efficacy and safety of eculizumab monotherapy in patients with AQP4+ NMOSD during PREVENT and/or its ongoing open-label extension (OLE; NCT02003144). Methods: During PREVENT and its OLE, adults with AQP4+ NMOSD received eculizumab (maintenance dose, 1200 mg/2 weeks) or placebo (PREVENT only) with/without concomitant IST. Relapses, hospitalizations, IST changes and adverse events (AEs) with eculizumab monotherapy (PREVENT and its OLE; interim data cut-off, July 31, 2019) or with placebo alone (PREVENT) were descriptively analyzed post hoc. Results: During PREVENT and/or its OLE, 33 patients received eculizumab monotherapy for a total of 85.3 patient-years (PY). Adjudicated relapses occurred in 1/33 patients (annualized relapse rate [ARR], 0.012; 95% confidence interval [CI]: 0.0020.082), vs 7/13 with placebo alone in PREVENT. At 192 weeks, 96.2% of patients who received eculizumab monotherapy were relapse-free (95% CI: 0.7570.994) vs 93.8% of patients who received eculizumab with concomitant IST (95% CI: 0.8670.972). No patients receiving eculizumab monotherapy required hospitalization for a relapse and none started an IST. The treatment-related AE rate with eculizumab monotherapy in PREVENT and its OLE was similar to that with placebo alone in PREVENT (181.0 and 186.0 events/100 PY, respectively), the infection rate was similar between these groups (174.1 vs 186.0 events/100 PY), and the treatment-related serious AE rate was lower with eculizumab monotherapy than with placebo alone (5.7 vs 23.3 events/100 PY). No meningococcal infections or deaths occurred among these patients. Conclusions: A very high proportion of patients who had experienced 12 relapses in the pre-study year remained relapse-free through 192 weeks of eculizumab monotherapy. Long-term eculizumab monotherapy was well tolerated. These data support the long-term effectiveness of eculizumab monotherapy in reducing relapse risk in AQP4+ NMOSD.