Background: Despite the availability of diagnostic criteria, an overlap between neuromyelitis optica (NMO) and multiple sclerosis (MS) exists, particularly in the early stage of the disease. Objective: To study the value of NMO-immunoglobulin G (IgG) determination in Caucasian patients with a first demyelinating episode who develop a relapsing form of optic neuritis or myelitis. Methods: This study was based on a prospectively acquired cohort of patients regarded as having a clinically isolated syndrome (CIS) at the time of presentation. From this cohort, 2 different groups were selected: group 1 (NMO phenotype), consisting of a first attack involving the optic nerve or the spinal cord, and at least a second event affecting either topography, and group 2 (negative control group), consisting of a first attack involving the brainstem or the cerebral hemispheres and at least 1 relapse in any topography. Group 3 was composed of patients with NMO according to the 2006 revised diagnostic criteria. Serum NMO-IgG was determined by indirect immunofluorescence. Results: A total of 3.1% of the group 1 patients were positive for NMO-IgG in comparison to 3.9% of group 2 and 44.5% of group 3, NMO. One of the positive patients in group 1 evolved to NMO. Conclusions: NMO-IgG determination is crucial in detecting patients who will develop NMO; however, its value as a routine test in cases presenting with symptoms of the type seen in MS is low, and should only be performed in those patients in which the initial diagnosis is not clear. Copyright © 2012 by AAN Enterprises, Inc.