It is widely accepted that cyano- and/or hydroxo-cobalamin can bring forth a complete remission of haematologic alterations in pernicious anemia, but the neurologic disorders can not necessarily be cured completely with either vitamin B_<12>. Since vitamin B_<12> is expected to be converted to coenzyme form, 5,6-dimethylbenzimidazolyl cobamide coenzyme, which is now believed to be a naturally occurring active vitamine B_<12>, it may be reasonable to assume that conenzyme B_<12> might be more effective for treatment of such neurologic disorders. With this in mind, therapeutic trial with coenzyme B_<12> was made in neurologic impairments associated with various diseases, in order to observe the effect on neurological symptoms. As a result, more than was expected, a marked therapeutic effect of coenzyme B_<12> was observed in 10 of 12 cases tested, most striking among which was a case of myelitis, in that cyano- and/or hydroxo-cobalamin was found ineffective despite massive doses of total 136 mg were parenterally given. This paper describes the results of this observation.