Neuromyelitis optica (NMO) is an inflammatory disease that selectively affects the optic nerves and spinal cord. The discovery of NMO-IgG targeting aquaporin-4 (AQP4) in NMO patients suggested that NMO is a distinct entity, with a fundamentally different etiology from that of multiple sclerosis (MS). Although NMO usually leads to grave disability because of the more severe tissue destruction compared with classical MS, there have been several reports describing a benign form of NMO over a long disease term. NMO-IgG/AQP4 antibodies show high specificity but medium sensitivity for NMO, while the clinical relevance of AQP4 antibody titers remains to be determined. We aimed to clarify the clinical relevance of AQP4 antibody levels determined by a bridging enzyme-linked immunosorbent assay in 38 patients with NMO or NMO spectrum disorder. The AQP4 antibody levels were higher in patients with optic neuritis (ON) than in those without ON (p = 0.0164). Among the 12 patients examined in a longitudinal study, four showed an increase in the ELISA values during some relapses, and eight showed no clear correlation between the ELISA values and relapse. Of the four patients who demonstrated a steady rise in the antibody levels over time, two patients had no concomitant relapses, despite elevation of the AQP4 antibody levels. We conclude that high AQP4 antibody levels are associated with the occurrence of ON, but that the antibody levels themselves are not closely correlated with the onset of relapse.