Ataxia telangiectasia (A-T) is a rare autosomal recessive disorder characterized by cerebellar ataxia, oculocutaneous telangiectasia, and variable degrees of humoral and cellular immunodeficiency. Affected individuals are known to exhibit a high incidence of lymphoma and leukemia. Because of increased chemosensitivity, the treatment of A-T patients with malignancies requires extremely careful planning and caution with respect to the use of chemotherapy. The authors report on a 12-year-old boy with A-T who developed B-cell lymphoma. He received a half-dose of the drugs administered according to the acute lymphoblastic leukemia (ALL) protocol issued by our children's cancer study group (9104 Standard Risk Protocol, Tokai Pediatric Oncology Study Group). As a result, he continues to be in complete remission and free of treatment complications 32 months after the diagnosis of B-cell lymphoma.