Ataxia telangiectasia (A-T) is a progressive neurological disorder with a birth incidence of approximately 1 in 300 000.1,2 The major neurological features include progressive cerebellar ataxia presenting in infancy, oculomotor dyspraxia and dysarthria. Immunodeficiency is an important feature of this disorder, although it is not usually severe. A majority of patients, if not all, have a deficiency of cell-mediated immunity, whereas deficiencies in humoral immunity are more variable. The resulting predisposition to infection is very variable between patients with some not noticeably affected and others showing frequent episodes of severe infection. In addition, patients show thymic hypoplasia, hypogonadism, a high level of serum alpha-fetoprotein (AFP), growth retardation and an abnormality of blood vessels (telangiectasia). The ATM gene also confers an increased radiosensitivity, which can be observed both in patients and in cultured cells from patients.