Chronic oxidative stress is involved in the pathogenesis of multiple inflammatory diseases, including cardiovascular disease and atherosclerosis. The rare autosomal recessive disorder Ataxia-telangiectasia (A-T) is characterized by progressive cerebellar ataxia secondary to Purkinje cell death, immunodeficiency, and increased cancer incidence. ATM, the protein mutated in A-T, plays a key role in cellular DNA-damage responses. A-T cells show poor cellular anti-oxidant defences and increased oxidant sensitivity compared to normal cells, and ATM functions, in part, as an oxidative stress sensor. The oxidation of low-density lipoprotein (oxLDL) and its uptake by macrophages is an initiating step in the development of atherosclerosis. We demonstrate that oxLDL activates ATM and downstream p21 expression in normal fibroblasts and endothelial cells. In ATM-deficient fibroblasts oxLDL induces DNA double-strand breaks, micronuclei formation and causes chromosome breaks. Furthermore, oxLDL decreases cell viability and inhibits colony formation in A-T fibroblasts more effectively as compared to normal controls. Formation of oxLDL-induced reactive oxygen species is significantly higher in A-T, than normal fibroblasts. Last, pre-treatment of cells with ammonium pyrrolidine dithiocarbamate, a potent antioxidant and inhibitor of transcription factor nuclear factor κB, reduces oxLDL-induced reactive oxygen species formation. Our data indicates that ATM functions in the defence against oxLDL-mediated cytotoxicity.