Background: Chronic intravenous immunoglobulin (IVIg) is used to treat refractory myasthenia gravis (MG). This subgroup analysis evaluated response to eculizumab in patients receiving chronic IVIg before entry to REGAIN, a phase 3, randomized, double-blind, placebo-controlled study of eculizumab in anti-acetylcholine receptor antibody-positive refractory generalized MG. Methods: IVIg was only permitted during REGAIN as rescue therapy; previously treated patients underwent a 4-week washout before randomization. Patients included in this analysis had received chronic IVIg ≥4 times in 1 year, with ≥1 dose within 6 months before REGAIN entry. Exacerbations and MG status changes were assessed. Results: Eighteen patients were evaluated; four experienced exacerbations (eculizumab-treated, 1/9; placebo-treated, 3/9). Clinically relevant improvements were larger with eculizumab than placebo, respectively (mean change, standard deviation [SD]: MG Activities of Daily Living score [MG-ADL], -5.3 [4.0] vs -2.1 [2.8]; Quantitative MG score [QMG], -4.1 [6.1] vs -1.3 [3.5]). More patients receiving eculizumab (7/9) had clinically meaningful responses (MG-ADL ≥3 and/or QMG ≥5 points) than those receiving placebo (3/9). Eculizumab safety was consistent with previous reports. Interim data from the open-label extension of REGAIN will be presented. Conclusions: In patients previously receiving chronic IVIg, eculizumab showed a trend toward meaningful clinical improvements and fewer exacerbations compared with placebo. (NCT01997229, NCT02301624).