Ataxia-telangiectasia (AT) is an uncommon genetic disorder characterized by cerebellar ataxia, oculocutaneous telangiectasias, progressive immunodeficiency, and a predisposition to lymphoid malignancy. The genetic defect in AT predisposes not only to malignancy but also to severe toxicity from anti-neoplastic therapies. It is important to consider the diagnosis of AT in any child with a lymphoid malignancy at a younger than expected age, or who has a pre-existing ataxia, to anticipate unusually severe toxicities from the antineoplastic therapy, to avoid confusing the development of ataxia with toxicity from therapy, and to provide appropriate genetic counseling. We describe two children at a young age with a lymphoid malignancy diagnosed before the diagnosis of AT. One patient had severe toxicity from his chemotherapy, requiring truncation of the planned course of treatment. The other child was able to tolerate his entire planned course of therapy, but ataxiathat was initially interpreted as toxicity from chemotherapy rather than as a sign of his AT developed. Lymphoid malignancy may be the presenting sign of AT. Making this diagnosis may influence therapy of the malignancy. The neurologic manifestations of the disease can be misinterpreted as toxicities of the chemotherapy, and diagnosis of AT allows appropriate genetic counseling for the family.