Ataxia-telangiectasia (A-T) is a genetic disorder caused by mutational inactivation of the ATM gene. A-T patients display a pleiotropic phenotype and suffer primarily from progressive ataxia caused by degeneration of cerebellar Purkinje and granule neurons. Disruption of the mouse Atm locus creates a murine model of A-T that exhibits most of the clinical features of the human disease. We previously hypothesized that some aspects of A-T, such as the preferential loss of certain neurons, could result from a continuous state of increased oxidative stress (G. Rotman and Y. Shiloh, Cancer Surv., 29: 285–304, 1997; G. Rotman and Y. Shiloh, BioEssays, 19: 911–917, 1997). The present work tests this hypothesis by analyzing markers of redox state in brains of Atm-deficient mice. We found alterations in the levels of thiol-containing compounds in Atm (−/−) brains, as well as significant changes in the activities of thioredoxin, catalase, and manganese superoxide dismutase in Atm (−/−) cerebella. These changes are indicative of increased levels of reactive oxygen species, which are seen primarily in the cerebellum of Atm-deficient mice. Our findings support the hypothesis that the absence of functional ATM results in oxidative stress, which may be an important cause of the degeneration of cerebellar neurons in A-T.