Seventy-seven cases of the optic-spinal form of multiple sclerosis (OSMS) were collected from 6 institutes in 3 cities of Japan, and the clinical and MRI features were analyzed. Two-thirds of the OSMS patients had longitudinally extensive spinal cord MRI lesions (LESL), and had clinical features similar to those of relapsing neuromyelitis optica which often causes severe disability. In contrast, OSMS patients without LESL tended to have milder disease and had some feature commonly seen in the conventional form of MS. The percentage of OSMS without LESL in total OSMS has recently been increasing. The present study suggests that LESL is crucially important for distinguishing the two subtypes of OSMS.