Ataxia telangiectasia (AT) is an inherited cerebellar degeneration syndrome often associated with immune deficiency, notably, lymphopenia, hypogammaglobulinemia, and cellular immune dysfunction. Although autoimmunity is a common feature of many congenital and acquired immune deficiencies, it has not generally been thought to be associated with AT. We report a 7-year-old boy with AT who developed acute idiopathic thrombocytopenic purpura while on subcutaneous immunoglobulin replacement therapy for hypogammaglobulinemia. He responded promptly to high-dose intravenous immunoglobulin. This case reinforces the notion that one must be observant for autoimmune hematologic conditions in any child with qualitative or quantitative deficiencies in cellular immunity.