Alexander's disease is a rare and fatal disorder of the central nervous system. It may appear at any age so three forms are delineated: infantile, juvenile and adult form. Alexander's disease inescapably leads to psychomotor retardation, progressive loss of nervous functions and characteristic changes in neuroimaging studies. The authors present a case of a 6-year-old girl, who was admitted to the Neurology Department after an episode of long-term vomiting, trismus and blurred speech. Computed tomography and magnetic resonance imaging of the brain showed characteristic changes of the white matter in the frontal lobes, which enabled us to make a preliminary diagnosis of Alexander's disease. The diagnosis was subsequently confirmed by molecular genetic testing of the gene encoding glial fibrillary acidic protein (GFAP). This article also presents clinical symptoms and course of this degenerative disorder. The authors point out the important role of neuroimaging and the necessity of molecular examination as a new diagnostic tool.