In 105 eyes with florid optic neuritis (ON) the sensation of brightness lessened when the frequency of flickering light was reduced to between 5 and 10 Hz, but remained stable or increased in 239 healthy eyes, 106 eyes after an ON had subsided, and 117 eyes with other diagnoses. This Aulhorn Flicker Test is highly accurate (98%) and specific. It gives pathologic results only for florid ON, and yields normal results again after the inflammation has subsided, even though other visual functions may remain reduced. The Flicker Test therefore makes it possible to distinguish between acute, recidivous, chronic and subsided ON, a distinction which is clinically often ambiguous and cannot be made on the basis of visually evoked potentials. The fact that the Flicker Test reveals pathologic conditions even in abortive cases or when an Uhthoff phenomenon is present is of great diagnostic value. Five representative cases from the group of patients with ON are described in some detail to show how the test can be used to judge the course of the disease, course, to predict its prognosis and to distinguish various patterns of development.