Longitudinally extensive transverse myelitis (LETM) is a syndrome with extensive spinal cord lesions spanning three or more vertebral segments on spinal cord MRI. Although many reports have indicated that LETM is a characteristic feature of neuoromyelitis optica (NMO) in Western countries, the clinical characteristics and risk for development of NMO in Korean patients with LETM is not clear. We retrospectively investigated the clinical, laboratory, radiological features, and prognosis of Korean patients with a first-ever episode of idiopathic LETM. Patients were classified into four subgroups, depending on their clinical course: monophasic LETM, recurrent LETM, NMO, and classic multiple sclerosis (MS). We compared various clinical, laboratory, and radiological features between groups. Of 20 patients with first-ever LETM, 15 (75%) were men, and 13 (65%) experienced clinical relapse over a mean follow-up period of 58 months. Three of 20 patients (two with NMO, one with recurrent myelitis) were seropositive for anti-AQP4 antibodies. The predominance of men in the monophasic and recurrent LETM groups compared to the NMO group was remarkable. In conclusion, Korean patients with LETM are predominantly male and have low seropositivity for anti-AQP4 antibody, which distinguishes them from LETM patients in Western countries. Patients with LETM and seropositivity for anti-AQP4 antibody have a high risk of relapse. The male predominance and the relatively low seropositive rate for anti-AQP4 suggests that rather than being a limited form of NMO, recurrent LETM is a new clinical entity in Koreans.