Alexander disease (AxD) is a leukodystrophy caused by heterozygous mutations in the gene for glial fibrillary acidic protein, an intermediate filament protein expressed by astrocytes. The mutation causes prominent protein aggregates inside astrocytes; there is also loss of myelin and oligodendrocytes and neuronal degeneration. We show that immunohistochemical staining for glutamate transporter 1, the major brain glutamate transporter expressed primarily in astrocytes suggests decreased levels in the hippocampi of infantile AxD patients. A knock-in mouse model of AxD also shows significant reduction of glutamate transporter 1 in the hippocampus. To explore this phenomenon at the cellular level, wild-type and R239C mutant glial fibrillary acidic proteins (the most common mutation) were overexpressed in astrocytes in culture. Western blotting and whole-cell patch clamp recordings demonstrated that the R239C astrocytes exhibited markedly reduced glutamate transporter 1 protein levels; this resulted in attenuated or abolished glutamate-induced inward transporter current. Neurons cocultured with the R239C astrocytes exhibited increased death after glutamate challenge. These results indicate that aberrant astrocytes have decreased glutamate uptake, which may play an important role in the pathogenesis of neuronal and oligodendrocyte injury and death in AxD.