Two cases of Alexander's disease are described. One case of infantile onset died at the age of 6 months and the second case was of the juvenile type with onset at 2 years and death at 10 years. A clinical diagnosis of this disease is difficult since signs can vary according to the age of the patient. The severity of the pathological changes can also depend upon the age of onset of this disease, but they are restricted to the central nervous system. The Rosenthal fibre is the characteristic feature of Alexander's disease and we have examined for the first time its ultrastructure and immunocytochemical characteristics at the electron microscopical level and demonstrated coexpression of anti-glial fibrillary acidic protein and anti-ubiquitin antisera.