AbstractAtaxia‐telangiectasia (A‐T), an inherited disorder giving radiation sensitivity and cancer‐proneness, is discussed in terms of a defect in ability to repair DNA damage. A new assay using damaged recombinant DNA molecules suggests that the fidelity of repair of DNA double‐strand breaks is reduced in an A‐T cell line. Specific chromosomal changes in some A‐T patients appear to be associated with cancer induction, and it is suggested that these could be linked to a DNA repair‐fidelity defect. However, a general correlation between radiosensitivity and cancer‐proneness is difficult to establish at present, partly because of diversity in radiosensitivity in the normal population.