A total of 66 samples (from 27 cases with neuromyelitis optica, 26 cases with multiple sclerosis, and 13 cases with optic neuritis) were tested for aquaporin-4 antibody by a cell-based immunofluores-cence assay and an enzyme-linked immunosorbent assay. The sensitivities and specificities of the two assays were similar. We further analyzed an additional 68 patients and 93 healthy controls using the enzyme-linked immunosorbent assay. A Kappa test showed good consistency between the two methods in terms of detection of anti-aquaporin-4 antibody in the sera of neuromyelitis optica patients. No significant correlations were identified with onset age or disease duration, suggesting that aquaporin-4 antibody is a good marker for neuromyelitis optica. The enzyme-linked immu-nosorbent assay can be used for quantifying aquaporin-4 antibody concentrations and may be useful to dynamically monitor changes in the levels of aquaporin-4 antibody during disease duration. A total of 66 samples (from 27 cases with neuromyelitis optica, 26 cases with multiple sclerosis, and 13 cases with optic neuritis) were tested for aquaporin-4 antibody by a cell-based immunofluorescence assay and an enzyme-linked immunosorbent assay. The sensitivities and specificities of the two assays were similar. We further analyzed an additional 68 patients and 93 healthy controls using the enzyme-linked immunosorbent assay. A Kappa test showed good consistency between the two methods in terms of detection of anti-aquaporin-4 antibody in the sera of neuromyelitis optica patients. No significant correlations were identified with onset age or disease duration, suggesting that aquaporin-4 antibody is a good marker for neuromyelitis optica. The enzyme-linked immu-nosorbent assay can be used for quantifying aquaporin- 4 antibody concentrations and may be useful to dynamically monitor changes in the levels of aquaporin-4 antibody during disease duration.