It is almost two hundred years since the description of the first neurological disorders in vitamin B12 deficiency. In spite of that, not all the aspects of the biological effect of the vitamin have been explained. It is well known, that the vitamin performs an important function in organic metabolism and two types of biochemical reactions depend on it. The haematological, gastrological and neurological disorders are the result of deficiency of that vitamin. This article deals with the neurological disorders--subacute degeneration changes of the spinal cord, polyneuropathy and psychical disorders--which accompany vitamin B12 deficiency. It also presents aetiology, epidemiology, clinical symptoms, pathomorphology and it describes the deviations which occur in accessory investigations. The authors pay special attention to the abnormalities which can occur in neurophysiological and imaging examinations made by neurologists. They concentrate on the problems of the differences, especially between multiple sclerosis and subacute degeneration. The article presents the treatment of neurological disorders with regard to different authors' opinions. The importance of the early diagnosis and treatment till regression of pathological symptoms has been particularly stressed.