Ataxia‐telangiectasia (A‐T) is an autosomal recessive disorder caused by mutations in the ATM gene. A‐T children demonstrate sensitivity to ionizing radiation, predisposition to hematological malignancies, and telangiectasias. However, the hallmark of A‐T is fulminant degeneration of cerebellar Purkinje cells accompanied by a progressive ataxia with features of both cerebellar and basal ganglia dysfunction. Although the ATM gene product (ATM) is known to be involved in DNA repair, the mechanisms that link loss of ATM with neurodegeneration remain unknown. Recently, it has been suggested that abnormalities in redox status contribute to the A‐T phenotype. To address this question in the nervous system, we measured reactive oxygen species (ROS) in brain regions and specific neuronal populations in ATM^−/− mice. We found increased ROS levels in cerebellum and striatum but not cortex of ATM^−/− mice compared to ATM^+/+ mice. Confocal microscopic examination revealed elevated superoxide levels in cerebellar Purkinje cells and nigral dopaminergic neurons but not cortical neurons, thus mapping increased superoxide levels onto the neuronal populations selectively affected in A‐T. These data are the first demonstration of elevated levels of ROS in neurons at risk in any genetic neurodegenerative disorder and, furthermore, suggest that ATM acts as a pro‐survival signal in post‐mitotic Purkinje cells and dopaminergic neurons by modifying superoxide radical handling in these selectively vulnerable neurons.