Ataxia telangiectasia (A-T) is a multisystem disease with diverse manifestations, including progressive neurodegeneration, immunodeficiency, respiratory disease, and genomic instability. One of the most important features of A-T is the increased predisposition to cancer, especially to lymphoid malignancies. Patients with A-T are generally excluded from collaborative clinical trials, their treatment outcomes and toxicity profiles have rarely been reported, and little is currently known concerning the treatment intensity required to provide a reasonable balance between efficacy and toxicity. The aims of this study are to build a large international de-identified database of children with A-T treated for leukemia and lymphoma, to investigate epidemiology and outcome of treatment, toxicity profiles and risk factors which impact outcome, in order to eventually enable the generation of data-based treatment recommendations for this population.