The complement system is essential in the pathogenesis of inflammatory central nervous system disorders. To investigate the involvement of complement pathways in neuromyelitis optica (NMO), levels of breakdown products for classical (C4d), alternative (FBb) and common (sC5b-9) pathways were measured in the sera of 28 NMO and control patients (30 Behcet's disease (BD), 29 multiple sclerosis (MS)) and 31 healthy controls by ELISA. Classical and/or alternative pathway consumption was enhanced in NMO and BD patients as compared to MS patients and healthy controls. Our results suggest that NBD and NMO differ from MS by the predominance of complement system involvement. (C) 2010 Elsevier B.V. All rights reserved.