Ten long-term survivors of childhood acute lymphoblastic leukemia were studied to determine if cytogenetic abnormalities were present in lymphocytes following discontinuation of therapy. The study included patients diagnosed between 1969 and 1974 who had received radiation therapy and a minimum of 3 years systemic chemotherapy. At study, the patients had been off all therapy from 1.3 to 6.5 years (median, 4 years). Peripheral blood lymphocytes were examined for spontaneous chromosome breakage and sister chromatid exchanges. In addition, G-banded metaphase and prometaphase chromosomes were analyzed. Chromosome breakage was found to be within normal limits for all patients. Likewise, there was no significant difference between patients and controls with respect to sister chromatid exchange frequency. However, seven of the ten patients were found to have one or more cells with nonclonal karyotypic abnormalities. Our results indicate that although long-term survivors of childhood acute lymphoblastic leukemia treated with intensive radiation and combination chemotherapy do not demonstrate chromosome instability or DNA damage as measured by breakage and sister chromatid exchange, a majority of these patients have a subpopulation of lymphocytes with nonclonal chromosome abnormalities years after stopping therapy.