Anti-aquaporin-4 (Aqp-4) antibody and complement system have emerged as major pathogenic factors in neuromyelitis optica (NMO). To test the significance of interleukin-6 (IL-6), another important humoral immunity factor, in NMO pathogenesis, we measured serum and cerebrospinal fluid (CSF) IL-6 levels of 23 NMO, 11 transverse myelitis, 16 optic neuritis, 27 relapsing remitting multiple sclerosis patients, and 20 neurologically normal controls. NMO and transverse myelitis patients had higher serum and CSF IL-6 levels than other groups. Particularly, anti-Aqp-4 positive NMO patients (n = 12) had higher serum/CSF IL-6 levels than anti-Aqp-4 negative patients (n = 11) and CSF IL-6 levels correlated with anti-Aqp-4 levels and disease severity of the NMO patients. Our results suggest that IL-6 is involved in NMO pathogenesis presumably via anti-Aqp-4 associated mechanisms.