Ataxia telangiectasia is a rare inherited and progressive neurological disorder in which patients show an unusual predisposition to T-cell leukaemia. We report here observations on a patient with a large cytogenetically abnormal clone showing a single t(X;14)(q28;q11) translocation which conferred a proliferative advantage on the cells. The further evolution of this clone to cytogenetically more complex clones of lymphocytes was seen in the patient. She subsequently developed a rapidly progressing T-cell leukaemia, with a CD4+CD8+ T-cell phenotype, about five years after the first appearance of additional chromosome translocations in the clone cells.