Lymphocytes from patients with ataxia telangiectasia and Down's syndrome show a greater frequency of chromosome aberrations after ionising radiation than do lymphocytes from normal people. The connection between Down's syndrome and Alzheimer's disease (AD) is well-known. In view of this and of a study showing a greater sensitivity of AD than of normal lymphoblastoid cells to X-irradiation (measured by viability), we have examined repair of AD lymphocytes (hydroxyurea-treated) by measuring [3H]thymidine incorporation after γ-irradiation. We have found no difference in incorporation between AD and normal lymphocytes from age-matched individuals. However, incorporation decreases with age in γ-irradiated cells and to a lesser extent in unirradiated cells.