SummaryDNA‐repair and the frequency of chromosome aberration after u.v. and X‐ray irradiation was studied on leukocytes from patients with Down's syndrome.The u.v.‐induced DNA‐repair synthesis was followed by the incorporation of [3H]thymidine in the presence of hydroxyurea. Similar dose‐response curves were estabIished for Down's syndrome leukocytes and controls. The cells from patients with Down's syndrome incorporated 70–75 % of the activity of control cells at the various doses (32–196 erg/mm.2). This difference was significant for the two highest u.v.‐doses (P<0.01). The yield of dicentric chromosomes after X‐ray exposure (150 rad.) was 35% higher in Down's syndrome leukocytes than in the contro