The autosomal recessive disorder ataxia-telangiectasia (AT) is highly pleiotropic. It is characterized by gradual loss of Purkinje cells in the cerebellum, leading to progressive neuromotor deterioration, immunodeficiency, developmental defects in specific tissues, profound predisposition to malignancy and acute sensitivity to ionizing radiation. AT cells show chromosomal instability, premature senesence, radiosensitivity and defects in cell cycle checkpoints activated by ionizing radiation. Several radiation induced pathways that regulate the cell cycle seem to be defective in AT cells, at least one of which is mediated by TP53. Extensive characterization of the cellular defects of AT cells, together with the recent isolation of the ATM gene, has provided some insight into the possible physiological roles of the ATM protein. Several lines of evidence, including the nature of the agents that elicit the hypersensitivity of AT cells, point to the possibility of a defect in the response to damage induced by oxidative stress, which affects various cellular macromolecules. The ATM protein might have a role in activating defence mechanisms against oxidative stress. This hypothesis broadens the previous concept of the AT defect and explains several aspects of the AT phenotype that cannot be accounted for by defective processing of DNA damage.