Introduction: Complement activation at the neuromuscular junction is a primary cause of acetylcholine receptor loss and failure of neuromuscular transmission in myasthenia gravis (MG). Eculizumab, a humanized monoclonal antibody, blocks the formation of terminal complement complex by specifically preventing the enzymatic cleavage of complement 5 (C5). Methods: This study was a randomized, double-blind, placebo-controlled, crossover trial involving 14 patients with severe, refractory generalized MG (gMG). Results: Six of 7 patients treated with eculizumab for 16 weeks (86%) achieved the primary endpoint of a 3-point reduction in the quantitative myasthenia gravis (QMG) score. Examining both treatment periods, the overall change in mean QMG total score was significantly different between eculizumab and placebo (P=0.0144). After assessing data obtained from all visits, the overall change in mean QMG total score from baseline was found to be significantly different between eculizumab and placebo (P<0.0001). Eculizumab was well tolerated. Conclusion: The data suggest that eculizumab may have a role in treating severe, refractory MG. © 2013 Wiley Periodicals, Inc.