The ability to repair damage to DNA was compared in 2 groups of patients having undergone treatment for leukemia, one of which developed secondary leukemia (SL), and the other without signs of secondary malignancy (treated controls). Both were related to normal controls. DNA repair was assessed in isolated peripheral lymphocytes from the patients by measuring the rejoining of strand breaks following alkylation damage to the lymphocytes or by measuring unscheduled DNA synthesis. Day-to-day variability in the assays was considerable, but findings were that 5 out of 7 SL patients had repair deficiencies as measured by their ability to rejoin strand breaks, and 5 out of 7 had increased unscheduled DNA synthesis compared to treated and normal controls. All patients with SL and 4 out of 8 treated controls had inherent strand breaks in their DNA as compared to the normal controls when measured by alkaline elution.