Longitudinal follow-up of MOG-seropositive patients of up to 6 years reveals fluctuating anti-MOG antibody levels. Longitudinal serum samples were available for two of the four anti-MOG antibody-positive patients. Serum samples were taken 30 or 29 years after disease onset over a time course of about 2 or 6 years (22 or 74 months). Anti-MOG reactivity is expressed as the geometric mean channel fluorescence (GMCF) ratio. The cutoff used (dotted line) is the mean GMCF ratio of the healthy donor group measured in parallel (n = 39) plus two standard deviations (cutoff = 1.45). The arrow indicates a clinical relapse. Antibodies against MOG fluctuated over the disease course: An increase of anti-MOG antibodies was associated in one patient (STB01) with an increased relapse frequency, but was independent of disease activity in the other patient (STB02).