A 26-year-old woman presented with headaches, incoordination and a cerebellar mass (1982). The CT scan revealed dilated ventricles and a hypodense space-occupying lesion adjacent to the fourth ventricle. Neuronal loss, gliosis and masses of Rosenthal fibers were seen in biopsy. There was no evidence of neoplasm. A second biopsy 2 years later was similar to the original specimen. A diagnosis of Alexander's disease was suggested. Later that year the patient's 11-year-old brother manifested a clinical picture initially diagnosed as brainstem glioma, but whose biopsy was characteristic of Alexander's disease. There has been a gradual deterioration of these siblings over the past 6 years (1986-1991). No evidence of neoplasm has appeared.