To diagnose neuromyelitis optica (NMO), the 2006 NMO diagnostic criteria is commonly used. However, adequate studies about the time course of NMO according to the criteria have been lacking. The aim of the study was to identify the interval between disease onset and diagnosis of NMO, as well as the clinical characteristics and time course, according to the 2006 NMO diagnostic criteria in Japanese patients with NMO. Clinical progression and time course of 43 Japanese patients with NMO who fulfilled the 2006 NMO diagnostic criteria with mean disease duration of 14.2 years were investigated retrospectively. The initial inflammatory event was myelitis in 44.2% (long extensive transverse myelitis [LETM] in 14.3%), optic neuritis in 41.9%, and concurrent myelitis and optic neuritis in 9.3% of the patients. The presence of LETM and anti-aquaporin-4 antibody seropositivity by the end of the observation period was found in 85.7 and 93.0% of the patients, respectively. Among the patients whose medical information were sufficiently available, the median intervals between NMO onset and the time until development of both optic neuritis and myelitis, LETM, or fulfillment of the 2006 NMO criteria were 16.5, 35.1, and 27.8 months, respectively. The development of diagnostic method at an early stage of NMO may be needed in order to initiate early treatment.