BACKGROUND BAFF (B-cell activating factor of the tumor necrosis factor family) and APRIL (a proliferation-inducing ligand) are two of the major survival factors for B cells. Many studies have shown that BAFF levels were elevated in MS patients. However, whether the levels of CSF BAFF/APRIL increased in NMO patients was still unclear. OBJECTIVE To measure the CSF BAFF and APRIL concentration of in NMO patients, and explore their relationship with disease activity in NMO. METHODS CSF BAFF and APRIL was measured by an enzyme-linked immunosorbent assay (ELISA) in NMO (n = 22), MS (n = 18) patients and controls (n = 14). RESULTS Concentration of BAFF and APRIL in NMO patients were significantly higher than MS and controls. CSF BAFF and APRIL levels in controls were also lower than MS. Both NMO and MS revealed an increased disease disability with increased CSF BAFF. CSF APRIL was associated with EDSS scores in NMO, but not found in MS. CONCLUSIONS BAFF/APRIL system considered important for aggressive B cells and T-cell responses, and may stimulates B cells and T cell activation in acute relapse of NMO and MS. In NMO patients, CSF BAFF and APRIL may be key factors of B cell immune response and reflect disease severity.